skill: expose the trust layer as a consumable peltier skill

The reusable artifact here was never the vendored targets — it is the
discipline: prove equivalence, prove significance, or claim nothing. This
packages it as a Claude Code skill usable from any repo.

Two modes. `verify` works anywhere, on any two shell commands in any
language: equivalence before timing -> A/A calibration -> interleaved A/B
with a bootstrap CI -> a verdict decided by the CI lower bound, never the
median. `attempt` drives the full in-repo loop (gates, pristine-baseline
verdict, ledger row, ROI report).

It drives the real bench-runner binary and refuses rather than degrade: on
an unsupported host, or with no trust layer reachable, preflight stops and
says so instead of falling back to a hand-rolled timing loop. A second copy
of the statistics is a second thing that can lie. A refusal ends the claim,
not the work.

An adversarial review of the skill caught six high-severity defects, all
fixed here — several of them the very errors the skill exists to prevent:

- verify mode had no risk classification at all. bench-runner emits only
  accepted/rejected-bench; needs-human-review is minted by the in-repo
  verdict path, which verify does not run. A concurrency change with
  byte-identical output would have sailed through as "accepted". The skill
  now makes the agent the classifier and says so explicitly.
- the accept template exposed `pin_prefix`, which is inert for
  compare/aa/calibrate (only `verdict` reads it) — an agent would have set
  it and reported pinning that never happened. Now documents wrapping both
  sides in taskset instead.
- the equivalence gate compared stdout only, so two commands that write to
  stderr and a dist/ tree would "GOLDEN PASS" on two empty files. Now hashes
  stdout + stderr + exit code + artifact tree, and an empty diff over empty
  output is a failed gate, not a passed one.
- nothing told the agent to build two coexisting immutable artifacts, so a
  `cargo run` baseline could rebuild from the patched tree mid-bench — the
  exact trap phase1-verdict-smoke-001 recorded.
- service mode was documented as generic HTTP. It is not: the server must
  take <port> <doc> <iters>, print READY, and close the connection. A stock
  Express app hangs the harness forever. Contract now stated.
- the CI refusal test could not pass: preflight also walks up from its own
  location, so running it in-place found this very repo. The test now copies
  the skill out of the tree first.

CI job `skill-preflight` asserts the contract that matters — resolve when a
trust layer is reachable, REFUSE when it is not, and work when consumed from
another repo via PELTIER_HOME. It deliberately does not assert a statistical
outcome: a shared runner is too noisy for that, which is precisely why the
skill calibrates before it claims.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude/skills/peltier/SKILL.md

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+---
+name: peltier
+description: Prove a performance change is real — or refuse to claim it. Use when optimizing code for speed, verifying a claimed speedup ("is this actually faster?", "did my optimization work?"), checking for a performance regression ("did this PR make things slower?"), or comparing the performance of two implementations. Runs peltier's trust layer: equivalence gates, A/A environment calibration, interleaved A/B with bootstrap confidence intervals. Never reports a naked percentage and never claims a win that is not statistically significant. Do NOT use for correctness-only work.
+---
+
+# peltier — prove it, or claim nothing
+
+A performance change that has not passed equivalence gates **and** shown a
+statistically significant improvement **does not exist**. You may discard it
+or log it. You may not claim it.
+
+Most benchmarking is wrong in ways that flatter the author: run-to-run noise
+read as a win, a faster binary that computes something subtly different, one
+lucky run out of five, a percentage with no interval attached. This skill
+exists to make those failures impossible rather than unlikely.
+
+## Preflight — never skip, never work around
+
+Run `scripts/preflight.sh` **relative to this skill's directory** (resolve its
+absolute path first — the skill may be installed at `~/.claude/skills/`, as a
+plugin, or copied into another project):
+
+    sh <skill-dir>/scripts/preflight.sh
+
+It locates the trust layer, builds the real `bench-runner`, and checks this
+host can run it. `STATUS=ok` does **not** mean the host can measure your
+change — only the A/A self-test decides that.
+
+**If it refuses, that ends the claim, not the work.** You may still write the
+patch and explain why you think it is faster. You may not attach a number to
+it. Do not fall back to `time`, `hyperfine`, or a stopwatch loop — a number
+you cannot stand behind is worse than no number, because someone will act on
+it. The statistics live in `bench-runner` and are used from there; a second
+copy of the math is a second thing that can lie.
+
+## Two modes
+
+| Mode | Use when | Read |
+|---|---|---|
+| **verify** | You have a baseline and a candidate — any repo, any language. | [reference/verify.md](reference/verify.md) |
+| **attempt** | You are inside a peltier checkout and want the full loop: profile → hypothesize → patch → gated verdict → ledger row. | [reference/attempt.md](reference/attempt.md) |
+
+Default to **verify**. It is the portable one, and it is what almost every
+request actually wants.
+
+### No candidate yet? Profile first — do not guess.
+
+"Measure before optimizing" is not advice, it is a gate. If you have one
+program and a hunch, get profile data *before* writing a patch: `perf record`
+(native), `py-spy` / `cProfile` (Python), `node --prof` / `--cpu-prof` (Node),
+`cargo flamegraph` (Rust), or `just coz` for causal profiling inside a peltier
+checkout. Then form a hypothesis about a *named* hot path, and only then patch.
+Guessing where the time goes is how people spend a week on 0.3% of the runtime.
+
+## The order of the gates is the point
+
+Each gate can end the job, and ending early is a success, not a failure.
+
+1. **Equivalence — before you time anything.** A faster program that does
+   something different is not an optimization, it is a bug with good
+   benchmarks. Prove baseline and candidate produce identical output on a fixed
+   workload. Comparing two *empty* outputs is a failed gate, not a passed one.
+2. **Calibration — can this machine measure this change at all?** The A/A
+   self-test runs the same command against itself and must find **nothing**. A
+   harness that "discovers" a speedup between a binary and itself is measuring
+   noise, and every number it gives you afterwards is fiction.
+3. **Measurement — interleaved A/B, bootstrap CI.** Not "run A five times, then
+   B five times" — that hands any thermal or cache drift to whichever side ran
+   last.
+4. **Risk — what did the patch actually touch?** Concurrency primitives,
+   `unsafe`/raw memory, floating-point reassociation: `needs-human-review`,
+   **regardless of how good the numbers are**. In verify mode *you* are the
+   classifier — `bench-runner` only ever emits `accepted` or `rejected-bench`,
+   so it cannot make this call for you.
+5. **Verdict — the CI lower bound decides.** Accept only if the bootstrap 95%
+   CI lower bound of the speedup clears the threshold (default 2%). The median
+   is not the claim. The lower bound is the claim.
+
+## Non-negotiables
+
+- **Cheapest wins first:** build flags → LTO → PGO → allocator swap → *then*
+  code changes. Do not hand-write SIMD before you have tried `-O3`.
+- **Every attempt is recorded, including failures.** A rejected hypothesis is a
+  result — it stops the next person from re-running your dead end.
+- **A failed gate is a complete, valid outcome.** Write it up and move on. Do
+  not iterate on a rejected patch more than twice without a new hypothesis.
+- **The thing being measured never grades itself.** The patch may not touch the
+  harness, the corpus, or the tests.
+
+## Never
+
+- **Never a naked percentage.** Every number carries its confidence interval
+  and the workload it was measured on. "12% faster" is not a result.
+- **Never re-run a benchmark until it passes.** That is p-hacking with extra
+  steps. The first calibrated run is the answer.
+- **Never claim a win the CI lower bound does not support**, however good the
+  median looks and however "obviously" faster the change is.
+- **Never present a measurement from a host that failed calibration.**
+
+## What good looks like
+
+**A claim, fully dressed** (`phase3-cjson-002`, in the ledger):
+
+> Accepted. Speedup 1.088 (candidate is 8.8% faster), 95% CI [1.074, 1.100] —
+> so the *defensible* claim is ≥7.4%, not 8.8%. Workload: 5.6 MB synthetic
+> JSON, 20k records, parsed + serialized 2×, single thread. Gates: upstream
+> suite green, golden replay byte-identical, ASan + UBSan clean.
+
+Note what that write-up does **not** say. It does not claim a differential-fuzz
+run, because that row records `fuzz_iters=0` — the fuzzing of that era ran
+out-of-band and the machine never witnessed it. Fuzz is a pipeline gate now, and
+new rows carry a real count. Say what the record supports, not what you
+remember doing.
+
+**Declining to claim what the data won't support** (`phase4-cjson-service`):
+p50 latency improved 6.2% [95% CI +5.8%, +7.2%] at 150 rps — claimed. The p99
+CI came back [0.07, 4.97] — uselessly wide, so **p99 was not claimed at all**,
+even though its median looked like a win.
+
+**Good numbers on a bad change** (`phase2-comrak-010`): 1.107 speedup, 95% CI
+[1.079, 1.121], on 11 MB of markdown. Tight interval, real improvement,
+**accepted — and wrong.** LeakSanitizer caught it afterwards and the row was
+overturned. It never shipped. The bench was never the problem; skipping a gate
+was. This is why the order above is not negotiable.

.claude/skills/peltier/assets/accept.template.toml

@@ -0,0 +1,41 @@
+# peltier accept thresholds — consumer template.
+#
+# Copy this next to the code you are measuring and pass it with
+# `bench-runner --config <this-file>`. The defaults are peltier's own
+# published bar; loosening them to make a result pass is not measurement,
+# it is negotiation.
+
+# The bootstrap CI *lower bound* of the speedup ratio must clear 1 + threshold.
+# The median is not the claim. The lower bound is the claim.
+threshold = 0.02
+
+# Measured runs per side, interleaved ABAB (SPEC minimum: 30).
+runs_per_side = 30
+
+# Warm-up runs per side, discarded — they pay for cold caches and JIT.
+warmup_runs = 3
+
+# Bootstrap resamples for the ratio CI.
+bootstrap_iters = 10000
+
+# Confidence level for the ratio CI.
+confidence = 0.95
+
+# Fixed PRNG seed so a calibration run is reproducible by anyone.
+bootstrap_seed = 1887343589
+
+# CPU pinning. WARNING: this key is INERT for `compare`, `aa`, and `calibrate`
+# — it is read only by the in-repo `verdict` pipeline. Setting it here and then
+# reporting "pinned to core N" would be a claim about a run that never happened.
+#
+# To actually pin a compare/aa/calibrate run, wrap BOTH sides identically in
+# the command itself:  --baseline 'taskset -c 2 <cmd>' --candidate 'taskset -c 2 <cmd>'
+#
+# Pinning tightens the A/A confidence interval ~5x on peltier's calibration
+# hardware. An unpinned host is not disqualified; a NOISY one is, and the A/A
+# self-test decides that empirically. Leave this empty.
+pin_prefix = ""
+
+# Differential-fuzz iterations requested per gate run. Only used by the
+# in-repo `verdict` pipeline, which requires a target to declare [gates].fuzz.
+# fuzz_iters = 10000

.claude/skills/peltier/reference/attempt.md

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+# attempt — the full loop, inside a peltier checkout
+
+Profile → hypothesize → patch → gated verdict → ledger row. This mode only
+applies when you are working **inside a peltier checkout** on a target that has
+a `targets/<name>/target.toml`. Everywhere else, use
+[verify.md](verify.md).
+
+Requires Linux (harnessd uses Unix sockets and `setsid`; every gate shells
+through `sh`).
+
+## Before you touch anything
+
+**Read the ledger first.** It records every attempt ever made on this target,
+including the failures — which is exactly what stops you re-running a dead
+end:
+
+    cargo run -p policy          # ranks playbook classes by their evidence
+
+`policy` reads the ledger and ranks optimization classes by the Wilson lower
+bound of their shippable-win rate. It is **advisory** — a learned prior over
+the fixed cheapest-first order. The gates still decide every verdict.
+
+**Then respect the order.** Cheapest wins first: build flags → LTO → PGO →
+allocator swap → *then* code changes. Do not hand-write SIMD before trying
+`-O3`. The playbook (`playbook/`) has the preconditions and known risks for
+each class.
+
+## The boundary you may not cross
+
+The agent may modify **only** allowlisted paths under `targets/<name>/`.
+`crates/`, `config/`, `corpora/`, and the upstream test suites are read-only:
+the thing being measured does not get to grade itself. `harnessd` enforces
+this on every proposed diff (path allowlist over every diff header, including
+`rename`/`copy` — a patch that tries to escape is rejected, not sanitized).
+
+## Log the hypothesis *before* patching
+
+A hypothesis written after seeing the numbers is not a hypothesis. State what
+you think is slow, why, and what you expect the change to buy — then patch.
+
+## Run the gates
+
+    just gates <target>
+
+Corpus pin → test-suite pin (when `corpora/<t>/TESTSUITE.sha256` exists) →
+upstream tests → golden replay. Note what this **does not** do: differential
+fuzz needs a pristine baseline to differ against, so it reports `Skipped` here
+and runs for real on the accept path below. `just gates` is a fast equivalence
+check, not a fuzz run — do not report it as one.
+
+## The verdict — the only thing that can accept a change
+
+    just verdict <target> <candidate-bin> <run-id> <class> "<hypothesis>" "<hotspot>" \
+        --patch-file <diff>
+
+This rebuilds a **pristine baseline** from the pinned upstream commit (not
+whatever is lying around in your tree), runs the full gate set against the
+candidate, benches interleaved A/B against that baseline, and writes one
+append-only ledger row with the verdict and the complete evidence.
+
+It will refuse to hand you an `accepted` unless:
+
+- the upstream test suite is green **and** golden replay is byte-identical (or
+  within the target's declared FP tolerance);
+- the **differential-fuzz gate actually passed** — a real run reporting zero
+  divergences. No fuzz run, no machine accept. Ever;
+- ASan/UBSan are clean (and TSan, when the target configures it);
+- the bootstrap 95% CI lower bound of the speedup clears the threshold in
+  `config/accept.toml` (default 2%);
+- and a conservative lexical risk classifier finds **no** concurrency, unsafe,
+  or floating-point signals in the patch. If it does, the verdict is capped at
+  `needs-human-review` no matter how good the numbers are.
+
+Any of these failing produces a rejected row. **That is a complete, valid
+outcome** — the row is the deliverable. Write it and move on. Do not iterate on
+a rejected patch more than twice without a genuinely new hypothesis.
+
+## Report
+
+    just report <run-id>
+
+Mechanically generates the ROI from the ledger row: throughput → cores →
+dollars, and/or latency percentiles — every figure with its CI, workload, and
+methodology printed inline. It flags any row that was not accepted, so a
+rejected attempt cannot be laundered into a dollar figure.
+
+## Unattended
+
+    just agent-attempt <target> <run-id>
+
+Runs one full attempt behind the OS boundary: `harnessd` as the trusted uid on
+a Unix socket, the agent loop as an unprivileged user, the verdict pipeline
+wrapped in a network namespace (`scripts/no-net.sh`) that **fails closed** — if
+namespaces are unavailable it refuses to run rather than quietly running with
+the network up. `just isolation-check` verifies the boundary from the agent's
+side.
+
+## When the numbers are good and the change is scary
+
+FP-ordering, concurrency primitives, anything UB-adjacent: `needs-human-review`,
+always, regardless of the speedup. This is not a formality. The one false
+accept in peltier's history (`phase2-comrak-010`) passed its bench and was
+caught by a human audit afterwards — which is precisely why the sanitizer lane
+is now machine-enforced and why this rule does not bend.

.claude/skills/peltier/reference/verify.md

@@ -0,0 +1,203 @@
+# verify — prove a speedup in any repo
+
+You have two things that should do the same work, one of which you think is
+faster. Find out. Works on any pair of shell commands, in any language.
+
+## 0. Preflight
+
+    sh <skill-dir>/scripts/preflight.sh
+
+Read the `key=value` output; you need `BENCH_RUNNER` and `PIN_SUPPORTED`. A
+refusal ends the **claim**, not the work — report the reason verbatim and do
+not substitute another timing tool.
+
+    BR=<BENCH_RUNNER from preflight>
+
+## 1. Config
+
+    cp <skill-dir>/assets/accept.template.toml ./peltier-accept.toml
+    CFG=./peltier-accept.toml
+
+Do **not** loosen `threshold`, `runs_per_side`, or `confidence` to make a
+result pass. That is not measurement, it is negotiation.
+
+**Pinning is not done through this file.** `pin_prefix` is read only by the
+in-repo `verdict` pipeline — `compare`, `aa`, and `calibrate` ignore it
+entirely (bench-runner's own fingerprint says `"caller-provided (wrap cmd in
+taskset)"`). If `PIN_SUPPORTED=yes` and you know a core is free, pin by
+wrapping **both sides identically**:
+
+    --baseline 'taskset -c 2 <cmd>' --candidate 'taskset -c 2 <cmd>'
+
+An unpinned host is not disqualified. A *noisy* one is, and step 3 is what
+decides that.
+
+## 2. Two immutable artifacts — not two states of one tree
+
+The harness runs the two sides **alternately, 66 times**. That is impossible if
+"baseline" and "candidate" are the same working tree in different states.
+Before measuring:
+
+- Build each side into its **own** location — a `git worktree` per side, or two
+  separate build/output directories. Never `git stash` between runs.
+- Pass the **built artifact paths** as the commands. Never pass a wrapper that
+  can rebuild: `cargo run`, `npm start`, `go run`, `mvn exec` will happily
+  rebuild from whatever is in the tree *mid-benchmark*, so you can end up
+  timing the same binary twice — and paying build cost inside the timed window.
+- Confirm the two artifacts actually differ: `sha256sum <baseline> <candidate>`.
+
+Peltier learned this the hard way (`phase1-verdict-smoke-001`: "workspace paths
+get silently rebuilt from the patched tree by test runs"), which is why the
+in-repo pipeline rebuilds a pristine baseline from the pinned upstream commit
+rather than trusting the tree.
+
+## 3. Equivalence — before you time anything
+
+Capture **everything** each side produces, on a fixed, representative workload:
+
+    run_side() {  # $1 = tag, $2 = command
+        rm -rf "out.$1"; mkdir -p "out.$1"
+        ( eval "$2" ) > "out.$1/stdout" 2> "out.$1/stderr"; echo "$?" > "out.$1/exit"
+    }
+    run_side base "<baseline-cmd>"
+    run_side cand "<candidate-cmd>"
+    diff -r out.base out.cand && echo "GOLDEN PASS" || echo "DIVERGENT"
+
+If the command's real output is a **file tree** (a bundler, a compiler, an
+image pipeline), hash that too — stdout is not the artifact:
+
+    ( cd dist && find . -type f | sort | xargs sha256sum ) > out.<tag>/tree
+
+**An empty diff over empty output is a failed gate, not a passed one.** If both
+sides wrote nothing to stdout, nothing to stderr, and you captured no artifact
+tree, you have compared nothing — stop and find the real output before going
+further. Then run the project's own test suite against the candidate; it must
+pass.
+
+- **Identical output + tests green** → proceed.
+- **Divergent** → stop. No speedup may be claimed from a change you cannot show
+  is behavior-preserving. The one exception — a change that *legitimately*
+  alters output, floating-point reassociation being the common case — is
+  `needs-human-review`, never an auto-accept. Quantify the divergence (how many
+  values, by how much) and hand it to a human.
+
+## 4. Calibration — can this host measure this change?
+
+The A/A self-test runs the **same command on both sides**. It must find nothing:
+
+    $BR --config "$CFG" aa --cmd '<baseline-cmd>'
+
+- stdout `A/A self-test passed (null verdict)` → the harness is not inventing
+  speedups. Proceed.
+- **Nonzero exit**, with `Error: A/A self-test FAILED: harness claims a speedup
+  from identical binaries` on **stderr** → stop. This host is too noisy to
+  measure this change; every number it produces afterwards is fiction. Report
+  the noise floor, then quiet the machine (pin a core, disable turbo/SMT, close
+  everything else) or move to calibrated hardware.
+
+For a claim anyone will act on, run the full calibration — a false-positive
+rate over many A/A sessions *plus* proof the harness can detect a **known
+injected regression**:
+
+    $BR --config "$CFG" calibrate --cmd '<baseline-cmd>' \
+        --sessions 20 --slowdown 0.05 --out calibration.json
+
+Required: **<5% A/A false-positive rate** and **≥95% detection** of the injected
+5% slowdown. A harness that cannot see a real 5% regression cannot be trusted
+to confirm a real 5% win. Keep `calibration.json` — it is the evidence that the
+measurement itself was sound.
+
+### Budget the runtime before you start
+
+`compare` executes the workload **(30 + 3) × 2 = 66 times**. `calibrate
+--sessions 20` executes it roughly **2,650 times**. Size the workload to
+**~0.2–2 s** of wall time: shorter and you measure process startup, longer and
+calibration takes hours. Run `calibrate` in the background — it will blow
+through a foreground command timeout.
+
+## 5. Measure — interleaved A/B, bootstrap CI
+
+    $BR --config "$CFG" compare --baseline '<baseline-cmd>' --candidate '<candidate-cmd>'
+
+Runs are **interleaved** (ABAB…), not blocked, so thermal drift and cache state
+hit both sides equally. Output:
+
+    speedup (baseline/candidate): median 1.0884, 95% CI [1.0745, 1.1001]
+    verdict: accepted
+
+The ratio is `baseline/candidate`, so **>1 means the candidate is faster**.
+
+`--perf-stat` (Linux) adds cycles/instructions/cache-miss medians — useful for
+explaining *why* something got faster, **never** part of the accept decision,
+and usually unavailable in VMs.
+
+## 6. Risk — you are the classifier here
+
+`bench-runner` emits only `accepted` or `rejected-bench`. It **cannot** route a
+dangerous change to review, because in verify mode nothing is inspecting your
+diff. So inspect it yourself, and override the tool:
+
+Read the patch. If it touches any of these, the outcome is **`needs-human-review`
+regardless of what `compare` printed**:
+
+- **Concurrency** — threads, locks, atomics, memory ordering, `volatile`, work
+  queues, async pools, `rayon`/`pthread`/`std::sync`.
+- **Unsafe / raw memory** — `unsafe`, pointer casts, `transmute`, FFI,
+  manual allocation, uninitialized memory.
+- **Floating-point reassociation** — reordering accumulation, `-ffast-math`,
+  FP SIMD, changed summation order.
+
+These changes can be *correct on your workload and wrong in production*, and a
+green bench says nothing about that. A byte-identical output does not clear
+them: a data race produces identical output right up until it doesn't.
+
+## 7. Verdict — read it honestly
+
+| What you see | What it means | What you may say |
+|---|---|---|
+| `verdict: accepted` | CI lower bound clears the threshold. | Claim it — with the **CI lower bound**, the interval, and the workload. |
+| `verdict: rejected-bench`, CI spans 1.0 (e.g. `[0.983, 1.001]`) | Indistinguishable from no change. | "No shippable win." A real, complete result. |
+| `verdict: rejected-bench`, CI **entirely below** 1.0 (e.g. `[0.91, 0.96]`) | A measured **regression**. | Report it as one, with the interval. Do not call it "no win." |
+| Any risk signal from step 6 | Out of the machine's hands. | `needs-human-review`. Never auto-accept. |
+
+Claim the **lower bound**, not the median: a median of 1.088 with CI
+[1.074, 1.100] supports "≥7.4% faster", not "8.8% faster".
+
+If the result is a rejection, **that is the answer**. Re-running until it passes
+is p-hacking. Two failed hypotheses on the same hot path means you need a new
+hypothesis, not another run.
+
+## Services (latency, not throughput)
+
+For a server, wall-clock-per-run is the wrong instrument. Service mode is an
+open-loop, fixed-rate load generator that measures each request from its
+**intended** send time, so queueing delay is counted rather than hidden
+(coordinated omission). It reports p50/p99 with bootstrap CIs.
+
+**It is not a generic HTTP load generator.** The server binary must:
+
+- take `<port> <doc> <iters>` as positional argv;
+- print a line beginning with `READY` on stdout, **flushed**, once listening;
+- answer `GET /` and **close the connection** (`Connection: close`) — the client
+  reads until EOF with no timeout, so a keep-alive server hangs the harness
+  forever.
+
+Point it at a stock Express or Flask app and it will either fail the READY
+handshake or block indefinitely. Peltier wrote a thin adapter binary
+(`targets/cjson/service.c`) for exactly this reason; you will need one too.
+
+    $BR --config "$CFG" service --baseline-bin '<bin>' --candidate-bin '<bin>' \
+        --doc <workload> --rate 150 --sessions 20
+
+Calibrate it the same way first (`service-calibrate`). Tail percentiles on a
+single-worker loopback setup are jittery — **if the p99 CI is wide, do not claim
+p99**, even when p50 is solid. Peltier's own shipped service result claims p50
+(+6.2%, CI [+5.8%, +7.2%]) and explicitly declines to claim p99 (CI [0.07, 4.97]).
+
+## Record the attempt — including the failures
+
+Outside peltier there is no ledger, so write the evidence where the next person
+will find it (a results file, the PR body): hypothesis, what changed,
+equivalence result, calibration evidence, the CI, the workload, and the verdict.
+**Failures are the most valuable rows** — they stop someone from re-running your
+dead end in six months.

.claude/skills/peltier/scripts/preflight.sh

@@ -0,0 +1,91 @@
+#!/bin/sh
+# peltier preflight — locate the trust layer, check this host can RUN it, and
+# build the real bench-runner.
+#
+# STATUS=ok means the harness is present and runnable here. It does NOT mean
+# this host can measure your change: only the A/A self-test decides that, and
+# it decides it empirically. Do not read STATUS=ok as "calibrated".
+#
+# A refusal (STATUS=refuse, nonzero exit) is a complete, valid outcome. It
+# ends the *claim*, not the work: you may still write and explain a patch, you
+# just may not attach a number to it. Never work around a refusal with a
+# hand-rolled timing loop.
+set -eu
+
+say() { printf '%s\n' "$1"; }
+refuse() {
+    say "STATUS=refuse"
+    say "REASON=$1"
+    exit 1
+}
+abspath() { ( CDPATH= cd -- "$1" 2>/dev/null && pwd ); }
+
+# --- host -------------------------------------------------------------
+# bench-runner shells every timed run through `sh -c`, and the in-repo
+# pipeline (harnessd, service mode, the sh-based gates) is POSIX-only.
+# Windows compiles the workspace but cannot run it.
+uname_s=$(uname -s 2>/dev/null || echo unknown)
+case "$uname_s" in
+    Linux)  host=linux ;;
+    Darwin) host=darwin ;;
+    *) refuse "unsupported host '$uname_s' — peltier's harness is Linux/POSIX-only at runtime. Run it on Linux (or macOS for verify mode). Do not substitute another benchmark." ;;
+esac
+
+# --- trust layer ------------------------------------------------------
+is_peltier() { [ -f "$1/crates/bench-runner/Cargo.toml" ] && [ -f "$1/config/accept.toml" ]; }
+
+# Walk up from a starting dir looking for a checkout. Absolute POSIX paths
+# always terminate at "/", so this cannot spin.
+walk_up() {
+    d=$1
+    while [ -n "$d" ] && [ "$d" != "/" ]; do
+        if is_peltier "$d"; then printf '%s' "$d"; return 0; fi
+        d=$(dirname "$d")
+    done
+    is_peltier / && printf '/'
+    return 0
+}
+
+home=""
+if [ -n "${PELTIER_HOME:-}" ]; then
+    canon=$(abspath "$PELTIER_HOME" || true)
+    [ -n "$canon" ] || refuse "PELTIER_HOME='$PELTIER_HOME' does not exist"
+    is_peltier "$canon" || refuse "PELTIER_HOME='$PELTIER_HOME' is not a peltier checkout (no crates/bench-runner/Cargo.toml + config/accept.toml)"
+    home=$canon
+else
+    # cwd first (working inside a checkout), then this script's own location
+    # (the skill still living inside one). A skill copied into another project
+    # finds neither — which is why PELTIER_HOME exists.
+    home=$(walk_up "$(pwd)")
+    [ -n "$home" ] || home=$(walk_up "$(abspath "$(dirname -- "$0")")")
+fi
+[ -n "$home" ] || refuse "no peltier checkout found — clone https://github.com/bdbrown4/peltier.io and set PELTIER_HOME=/path/to/peltier.io. The statistics live in bench-runner; do not reimplement them."
+
+command -v cargo >/dev/null 2>&1 || refuse "cargo not on PATH — needed to build bench-runner from $home"
+
+# --- build the real harness -------------------------------------------
+# Always build. cargo is a no-op when current, and skipping this because a
+# binary happens to exist would silently measure with a stale copy of the
+# statistics after a `git pull`.
+br="$home/target/release/bench-runner"
+( cd "$home" && cargo build --release -q -p bench-runner ) \
+    || refuse "bench-runner failed to build in $home — fix the build before claiming any number"
+{ [ -f "$br" ] && [ -x "$br" ]; } || refuse "bench-runner did not appear at $br after a successful build"
+
+# --- pinning ----------------------------------------------------------
+# NOTE: config/accept.toml's `pin_prefix` is read ONLY by the in-repo verdict
+# pipeline. For compare/aa/calibrate you must wrap the commands yourself
+# (`taskset -c N <cmd>` on both sides). PIN_SUPPORTED just says whether that
+# tool is available here.
+if [ "$host" = linux ] && command -v taskset >/dev/null 2>&1; then
+    pin=yes
+else
+    pin=no
+fi
+
+say "STATUS=ok"
+say "PELTIER_HOME=$home"
+say "BENCH_RUNNER=$br"
+say "HOST=$host"
+say "PIN_SUPPORTED=$pin"
+say "NOTE=harness runnable here; the A/A self-test decides whether it can measure your change"