skill: cross-OS preflight — ps1 entry point, one CI-enforced contract

On a Windows host the preflight's job is to deliver a structured refusal
(measurement is Linux/POSIX-only). But a Windows-native harness with no
POSIX shell never got that far: preflight.sh fails to LAUNCH, and an agent
staring at "sh: not found" is exactly the agent that hand-rolls a timing
loop. The refusal doctrine has to be as portable as the skill.

- scripts/preflight.ps1: faithful port of the preflight contract, runs on
  Windows PowerShell 5.1 and pwsh 7+ on any OS. Host check first (so
  Windows refuses before touching anything), same walk-up resolution, same
  always-build rule, same key=value output, same exit codes. The source is
  pure ASCII with the shared em dashes composed at runtime — Windows
  PowerShell 5.1 misdecodes BOM-less UTF-8, and that exact bug class
  already bit this machine once (the francis-watchdog parse failure).
- Two implementations of one contract is a drift risk, so it is gated like
  every other claim in this repo: the skill-preflight CI job now runs BOTH
  scripts on the same ubuntu host and byte-diffs their stdout on all three
  paths (in-checkout resolve, no-checkout refusal, PELTIER_HOME from a
  foreign repo). Disagreement is a red build, not a footnote.
- New skill-preflight-windows job (windows-latest): asserts the native
  refusal end-to-end under Windows PowerShell 5.1 AND pwsh (STATUS=refuse,
  nonzero exit, the 'unsupported host' reason), plus Git Bash sh for good
  measure. Verified live on a real Windows 11 box before shipping: 5.1
  parses the pure-ASCII source clean and refuses with exit 1.
- SKILL.md / references/verify.md: document the second entry point ("use
  whichever your host can launch; never both, never neither"); book chapter
  and CLAUDE.md updated to match. .agents mirror regenerated (identical).
  The ZeroClaw variant is unaffected: rm -rf scripts/ drops the ps1 too,
  and its audit still passes (7 files scanned).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.agents/skills/peltier/SKILL.md

@@ -30,6 +30,13 @@ plugin, or copied into another project):
 
     sh <skill-dir>/scripts/preflight.sh
 
+    # no POSIX shell on this host (Windows-native harness)? same contract:
+    powershell -NoProfile -ExecutionPolicy Bypass -File <skill-dir>/scripts/preflight.ps1
+
+The two entry points implement one contract and are byte-diffed against each
+other in CI — same checks, same output, same refusals. Use whichever your
+host can launch; never both, never neither.
+
 Some harnesses block script files inside skills, so your installation may
 carry no `scripts/` directory. The script still exists exactly once — in the
 peltier checkout this skill drives (which you need anyway; it is where

.agents/skills/peltier/references/verify.md

@@ -6,7 +6,9 @@ faster. Find out. Works on any pair of shell commands, in any language.
 ## 0. Preflight
 
     sh <skill-dir>/scripts/preflight.sh
-    # or, if this installation carries no scripts/ (script-blocking harness):
+    # no POSIX shell (Windows-native harness)? same contract, CI-diffed against the sh version:
+    powershell -NoProfile -ExecutionPolicy Bypass -File <skill-dir>/scripts/preflight.ps1
+    # installation carries no scripts/ (script-blocking harness)? run from the checkout:
     sh "$PELTIER_HOME/.claude/skills/peltier/scripts/preflight.sh"
 
 Read the `key=value` output; you need `BENCH_RUNNER` and `PIN_SUPPORTED`. A

.agents/skills/peltier/scripts/preflight.ps1

@@ -0,0 +1,112 @@
+# peltier preflight - PowerShell entry point. Same contract as preflight.sh,
+# for hosts where no POSIX shell exists (Windows-native harnesses). Works on
+# Windows PowerShell 5.1 and pwsh 7+ on any OS.
+#
+# THE CONTRACT IS DEFINED ONCE: this script and preflight.sh must emit
+# byte-identical output for the same environment. CI enforces that by running
+# both on the same host and diffing stdout (skill-preflight job). If you
+# change one, change the other, or the build goes red.
+#
+# 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.
+# A refusal (STATUS=refuse, exit 1) ends the *claim*, not the work.
+#
+# Source is deliberately pure ASCII: Windows PowerShell 5.1 misdecodes
+# BOM-less UTF-8, so the em dashes in the shared refusal strings are composed
+# at runtime instead of written literally.
+
+$ErrorActionPreference = 'Stop'
+$EM = [string][char]0x2014   # em dash, byte-identical to preflight.sh output
+
+function Say([string]$s) { Write-Output $s }
+function Refuse([string]$reason) {
+    Say 'STATUS=refuse'
+    Say "REASON=$reason"
+    exit 1
+}
+
+# --- host -------------------------------------------------------------
+# bench-runner shells every timed run through `sh -c`, and the in-repo
+# pipeline is POSIX-only. Windows compiles the workspace but cannot run it.
+# Host check comes FIRST, exactly as in preflight.sh.
+$onLinux = (Test-Path variable:IsLinux) -and $IsLinux
+$onMac   = (Test-Path variable:IsMacOS) -and $IsMacOS
+if ($onLinux) { $hostKind = 'linux' }
+elseif ($onMac) { $hostKind = 'darwin' }
+else {
+    Refuse ("unsupported host 'Windows' $EM peltier's harness is Linux/POSIX-only at runtime. " +
+            "Run it on Linux (or macOS for verify mode). Do not substitute another benchmark.")
+}
+
+# --- trust layer ------------------------------------------------------
+function Test-Peltier([string]$dir) {
+    (Test-Path (Join-Path $dir 'crates/bench-runner/Cargo.toml') -PathType Leaf) -and
+    (Test-Path (Join-Path $dir 'config/accept.toml') -PathType Leaf)
+}
+
+# Walk up from a starting dir looking for a checkout; parents terminate at
+# the filesystem root, so this cannot spin.
+function Find-Peltier([string]$start) {
+    $d = $start
+    while (-not [string]::IsNullOrEmpty($d)) {
+        if (Test-Peltier $d) { return $d }
+        $parent = Split-Path -Parent $d
+        if ($parent -eq $d) { break }
+        $d = $parent
+    }
+    return $null
+}
+
+$peltierHome = $null
+if ($env:PELTIER_HOME) {
+    $canon = $null
+    try { $canon = (Resolve-Path -LiteralPath $env:PELTIER_HOME).Path } catch {}
+    if (-not $canon) { Refuse "PELTIER_HOME='$($env:PELTIER_HOME)' does not exist" }
+    if (-not (Test-Peltier $canon)) {
+        Refuse "PELTIER_HOME='$($env:PELTIER_HOME)' is not a peltier checkout (no crates/bench-runner/Cargo.toml + config/accept.toml)"
+    }
+    $peltierHome = $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.
+    $peltierHome = Find-Peltier (Get-Location).Path
+    if (-not $peltierHome) { $peltierHome = Find-Peltier $PSScriptRoot }
+}
+if (-not $peltierHome) {
+    Refuse ("no peltier checkout found $EM 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.")
+}
+
+if (-not (Get-Command cargo -ErrorAction SilentlyContinue)) {
+    Refuse "cargo not on PATH $EM needed to build bench-runner from $peltierHome"
+}
+
+# --- build the real harness -------------------------------------------
+# Always build. cargo is a no-op when current, and skipping this because a
+# binary happens to exist would silently measure with a stale copy of the
+# statistics after a `git pull`.
+$br = Join-Path $peltierHome 'target/release/bench-runner'
+Push-Location $peltierHome
+try { & cargo build --release -q -p bench-runner; $buildOk = ($LASTEXITCODE -eq 0) }
+finally { Pop-Location }
+if (-not $buildOk) {
+    Refuse "bench-runner failed to build in $peltierHome $EM fix the build before claiming any number"
+}
+if (-not (Test-Path $br -PathType Leaf)) {
+    Refuse "bench-runner did not appear at $br after a successful build"
+}
+
+# --- pinning ----------------------------------------------------------
+# NOTE: config/accept.toml's `pin_prefix` is read ONLY by the in-repo verdict
+# pipeline. For compare/aa/calibrate you must wrap the commands yourself
+# (`taskset -c N <cmd>` on both sides). PIN_SUPPORTED just says whether that
+# tool is available here.
+if ($onLinux -and (Get-Command taskset -ErrorAction SilentlyContinue)) { $pin = 'yes' } else { $pin = 'no' }
+
+Say 'STATUS=ok'
+Say "PELTIER_HOME=$peltierHome"
+Say "BENCH_RUNNER=$br"
+Say "HOST=$hostKind"
+Say "PIN_SUPPORTED=$pin"
+Say 'NOTE=harness runnable here; the A/A self-test decides whether it can measure your change'

.claude/skills/peltier/SKILL.md

@@ -30,6 +30,13 @@ plugin, or copied into another project):
 
     sh <skill-dir>/scripts/preflight.sh
 
+    # no POSIX shell on this host (Windows-native harness)? same contract:
+    powershell -NoProfile -ExecutionPolicy Bypass -File <skill-dir>/scripts/preflight.ps1
+
+The two entry points implement one contract and are byte-diffed against each
+other in CI — same checks, same output, same refusals. Use whichever your
+host can launch; never both, never neither.
+
 Some harnesses block script files inside skills, so your installation may
 carry no `scripts/` directory. The script still exists exactly once — in the
 peltier checkout this skill drives (which you need anyway; it is where

.claude/skills/peltier/references/verify.md

@@ -6,7 +6,9 @@ faster. Find out. Works on any pair of shell commands, in any language.
 ## 0. Preflight
 
     sh <skill-dir>/scripts/preflight.sh
-    # or, if this installation carries no scripts/ (script-blocking harness):
+    # no POSIX shell (Windows-native harness)? same contract, CI-diffed against the sh version:
+    powershell -NoProfile -ExecutionPolicy Bypass -File <skill-dir>/scripts/preflight.ps1
+    # installation carries no scripts/ (script-blocking harness)? run from the checkout:
     sh "$PELTIER_HOME/.claude/skills/peltier/scripts/preflight.sh"
 
 Read the `key=value` output; you need `BENCH_RUNNER` and `PIN_SUPPORTED`. A

.claude/skills/peltier/scripts/preflight.ps1

@@ -0,0 +1,112 @@
+# peltier preflight - PowerShell entry point. Same contract as preflight.sh,
+# for hosts where no POSIX shell exists (Windows-native harnesses). Works on
+# Windows PowerShell 5.1 and pwsh 7+ on any OS.
+#
+# THE CONTRACT IS DEFINED ONCE: this script and preflight.sh must emit
+# byte-identical output for the same environment. CI enforces that by running
+# both on the same host and diffing stdout (skill-preflight job). If you
+# change one, change the other, or the build goes red.
+#
+# 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.
+# A refusal (STATUS=refuse, exit 1) ends the *claim*, not the work.
+#
+# Source is deliberately pure ASCII: Windows PowerShell 5.1 misdecodes
+# BOM-less UTF-8, so the em dashes in the shared refusal strings are composed
+# at runtime instead of written literally.
+
+$ErrorActionPreference = 'Stop'
+$EM = [string][char]0x2014   # em dash, byte-identical to preflight.sh output
+
+function Say([string]$s) { Write-Output $s }
+function Refuse([string]$reason) {
+    Say 'STATUS=refuse'
+    Say "REASON=$reason"
+    exit 1
+}
+
+# --- host -------------------------------------------------------------
+# bench-runner shells every timed run through `sh -c`, and the in-repo
+# pipeline is POSIX-only. Windows compiles the workspace but cannot run it.
+# Host check comes FIRST, exactly as in preflight.sh.
+$onLinux = (Test-Path variable:IsLinux) -and $IsLinux
+$onMac   = (Test-Path variable:IsMacOS) -and $IsMacOS
+if ($onLinux) { $hostKind = 'linux' }
+elseif ($onMac) { $hostKind = 'darwin' }
+else {
+    Refuse ("unsupported host 'Windows' $EM peltier's harness is Linux/POSIX-only at runtime. " +
+            "Run it on Linux (or macOS for verify mode). Do not substitute another benchmark.")
+}
+
+# --- trust layer ------------------------------------------------------
+function Test-Peltier([string]$dir) {
+    (Test-Path (Join-Path $dir 'crates/bench-runner/Cargo.toml') -PathType Leaf) -and
+    (Test-Path (Join-Path $dir 'config/accept.toml') -PathType Leaf)
+}
+
+# Walk up from a starting dir looking for a checkout; parents terminate at
+# the filesystem root, so this cannot spin.
+function Find-Peltier([string]$start) {
+    $d = $start
+    while (-not [string]::IsNullOrEmpty($d)) {
+        if (Test-Peltier $d) { return $d }
+        $parent = Split-Path -Parent $d
+        if ($parent -eq $d) { break }
+        $d = $parent
+    }
+    return $null
+}
+
+$peltierHome = $null
+if ($env:PELTIER_HOME) {
+    $canon = $null
+    try { $canon = (Resolve-Path -LiteralPath $env:PELTIER_HOME).Path } catch {}
+    if (-not $canon) { Refuse "PELTIER_HOME='$($env:PELTIER_HOME)' does not exist" }
+    if (-not (Test-Peltier $canon)) {
+        Refuse "PELTIER_HOME='$($env:PELTIER_HOME)' is not a peltier checkout (no crates/bench-runner/Cargo.toml + config/accept.toml)"
+    }
+    $peltierHome = $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.
+    $peltierHome = Find-Peltier (Get-Location).Path
+    if (-not $peltierHome) { $peltierHome = Find-Peltier $PSScriptRoot }
+}
+if (-not $peltierHome) {
+    Refuse ("no peltier checkout found $EM 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.")
+}
+
+if (-not (Get-Command cargo -ErrorAction SilentlyContinue)) {
+    Refuse "cargo not on PATH $EM needed to build bench-runner from $peltierHome"
+}
+
+# --- build the real harness -------------------------------------------
+# Always build. cargo is a no-op when current, and skipping this because a
+# binary happens to exist would silently measure with a stale copy of the
+# statistics after a `git pull`.
+$br = Join-Path $peltierHome 'target/release/bench-runner'
+Push-Location $peltierHome
+try { & cargo build --release -q -p bench-runner; $buildOk = ($LASTEXITCODE -eq 0) }
+finally { Pop-Location }
+if (-not $buildOk) {
+    Refuse "bench-runner failed to build in $peltierHome $EM fix the build before claiming any number"
+}
+if (-not (Test-Path $br -PathType Leaf)) {
+    Refuse "bench-runner did not appear at $br after a successful build"
+}
+
+# --- pinning ----------------------------------------------------------
+# NOTE: config/accept.toml's `pin_prefix` is read ONLY by the in-repo verdict
+# pipeline. For compare/aa/calibrate you must wrap the commands yourself
+# (`taskset -c N <cmd>` on both sides). PIN_SUPPORTED just says whether that
+# tool is available here.
+if ($onLinux -and (Get-Command taskset -ErrorAction SilentlyContinue)) { $pin = 'yes' } else { $pin = 'no' }
+
+Say 'STATUS=ok'
+Say "PELTIER_HOME=$peltierHome"
+Say "BENCH_RUNNER=$br"
+Say "HOST=$hostKind"
+Say "PIN_SUPPORTED=$pin"
+Say 'NOTE=harness runnable here; the A/A self-test decides whether it can measure your change'

.github/workflows/ci.yml

@@ -186,6 +186,31 @@ jobs:
             sh "$GITHUB_WORKSPACE/.claude/skills/peltier/scripts/preflight.sh" | tee pf.txt
           grep -q '^STATUS=ok$' pf.txt
 
+      - name: sh and ps1 preflights are byte-equivalent (two entry points, one contract)
+        run: |
+          # preflight.ps1 exists for hosts with no POSIX shell. Two
+          # implementations of the same contract can drift, so both run here
+          # on the same host and their stdout is byte-diffed on every path.
+          # 1. in-checkout resolve
+          sh .claude/skills/peltier/scripts/preflight.sh > /tmp/eq-sh.txt
+          pwsh -NoProfile -File .claude/skills/peltier/scripts/preflight.ps1 > /tmp/eq-ps.txt
+          diff /tmp/eq-sh.txt /tmp/eq-ps.txt
+          # 2. refusal with no checkout reachable (copied out of the repo)
+          cp -r .claude/skills/peltier /tmp/eqskill
+          mkdir -p /tmp/eq-not-a-repo && cd /tmp/eq-not-a-repo
+          set +e
+          env -u PELTIER_HOME sh /tmp/eqskill/scripts/preflight.sh > sh.txt 2>&1; shrc=$?
+          env -u PELTIER_HOME pwsh -NoProfile -File /tmp/eqskill/scripts/preflight.ps1 > ps.txt 2>&1; psrc=$?
+          set -e
+          test "$shrc" -ne 0 && test "$psrc" -ne 0
+          diff sh.txt ps.txt
+          # 3. resolve via PELTIER_HOME from a foreign repo
+          mkdir -p /tmp/eq-foreign && cd /tmp/eq-foreign
+          PELTIER_HOME="$GITHUB_WORKSPACE" sh /tmp/eqskill/scripts/preflight.sh > sh2.txt
+          PELTIER_HOME="$GITHUB_WORKSPACE" pwsh -NoProfile -File /tmp/eqskill/scripts/preflight.ps1 > ps2.txt
+          diff sh2.txt ps2.txt
+          echo "equivalence holds on all three paths"
+
       - name: The harness the skill drives actually runs (plumbing, not verdict)
         run: |
           BR=$(sed -n 's/^BENCH_RUNNER=//p' /tmp/pf.txt)
@@ -200,3 +225,40 @@ jobs:
         run: |
           BR=$(sed -n 's/^BENCH_RUNNER=//p' /tmp/pf.txt)
           "$BR" --config /tmp/accept.toml aa --cmd 'sleep 0.02'
+
+  # The refusal doctrine must survive hosts with no POSIX shell: on a real
+  # Windows runner, BOTH preflight entry points must refuse with a structured
+  # STATUS=refuse — never a launch failure an agent could misread as license
+  # to hand-roll a timing loop. Windows PowerShell 5.1 is tested separately
+  # from pwsh because 5.1 misdecodes BOM-less UTF-8 (the ps1 source is pure
+  # ASCII for exactly that reason).
+  skill-preflight-windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: ps1 refuses natively (Windows PowerShell 5.1)
+        shell: powershell
+        run: |
+          $out = & powershell -NoProfile -ExecutionPolicy Bypass -File .claude\skills\peltier\scripts\preflight.ps1
+          $out | Write-Output
+          if ($LASTEXITCODE -eq 0) { throw "preflight did not refuse on Windows" }
+          $joined = $out -join "`n"
+          if ($joined -notlike "*STATUS=refuse*") { throw "no STATUS=refuse in output" }
+          if ($joined -notlike "*unsupported host 'Windows'*") { throw "missing refusal reason" }
+      - name: ps1 refuses natively (pwsh)
+        shell: pwsh
+        run: |
+          $out = & pwsh -NoProfile -File .claude\skills\peltier\scripts\preflight.ps1
+          $out | Write-Output
+          if ($LASTEXITCODE -eq 0) { throw "preflight did not refuse under pwsh" }
+          if (($out -join "`n") -notlike "*STATUS=refuse*") { throw "no STATUS=refuse in output" }
+      - name: sh entry point also refuses here (Git Bash)
+        shell: bash
+        run: |
+          set +e
+          sh .claude/skills/peltier/scripts/preflight.sh > out.txt 2>&1
+          rc=$?
+          set -e
+          cat out.txt
+          test "$rc" -ne 0
+          grep -q '^STATUS=refuse$' out.txt

CLAUDE.md

@@ -29,7 +29,7 @@ Full engineering spec: `SPEC.md`. Read it before starting any phase.
 - `targets/` — vendored OSS targets for case studies (permissive licenses only)
 - `playbook/` — optimization classes, ordered, with preconditions and known risks
 - `results/` — generated reports and flamegraphs
-- `.claude/skills/peltier/` — the trust layer packaged as a consumable **Agent Skill** (agentskills.io SKILL.md standard, spec-compliant frontmatter): `verify` mode (equivalence → A/A calibration → interleaved A/B + bootstrap CI) works in any repo against any two shell commands; `attempt` mode drives the full in-repo loop. Drives the real `bench-runner` binary and refuses on unsupported hosts rather than falling back to a second, forkable copy of the statistics. Cross-harness: mirrored byte-identically at `.agents/skills/peltier/` (the Codex/Zed/Cursor/Gemini/Copilot convergence path — CI enforces the mirror), `AGENTS.md` at the repo root points non-Claude harnesses here, and `scripts/install-skill.sh` stamps consumer repos (plus a script-less `--zeroclaw-variant`, since ZeroClaw blocks script files in skills by default — its preflight runs from the checkout). Preflight refusal, spec conformance, mirror equality, and the installer are all asserted in CI (`skill-preflight`).
+- `.claude/skills/peltier/` — the trust layer packaged as a consumable **Agent Skill** (agentskills.io SKILL.md standard, spec-compliant frontmatter): `verify` mode (equivalence → A/A calibration → interleaved A/B + bootstrap CI) works in any repo against any two shell commands; `attempt` mode drives the full in-repo loop. Drives the real `bench-runner` binary and refuses on unsupported hosts rather than falling back to a second, forkable copy of the statistics. Cross-harness: mirrored byte-identically at `.agents/skills/peltier/` (the Codex/Zed/Cursor/Gemini/Copilot convergence path — CI enforces the mirror), `AGENTS.md` at the repo root points non-Claude harnesses here, and `scripts/install-skill.sh` stamps consumer repos (plus a script-less `--zeroclaw-variant`, since ZeroClaw blocks script files in skills by default — its preflight runs from the checkout). Preflight ships as two entry points implementing one contract — `preflight.sh` (POSIX) and `preflight.ps1` (Windows PowerShell 5.1/pwsh; pure-ASCII source, em dashes composed at runtime) — CI byte-diffs their output on all three paths and a `windows-latest` job asserts the native refusal. Preflight refusal, sh↔ps1 equivalence, spec conformance, mirror equality, and the installer are all asserted in CI (`skill-preflight`, `skill-preflight-windows`).
 
 **Phase 4 COMPLETE — all SPEC §5 exit criteria met. Phases 0–4 are all done.** Service-mode latency benchmarking shipped: a coordinated-omission-correct, open-loop, fixed-rate load generator (`crates/bench-runner/src/service.rs`) measuring latency from each request's intended send time (queueing counted, not hidden), interleaved A/B, exact percentiles, bootstrap p50/p99 CIs. A real service target — `targets/cjson/service.c`, a minimal HTTP server wrapping the patched `cJSON.c` (trust-layer, outside the workspace allowlist). Service calibration PASS (0/10 A/A false-positive, 10/10 injected-5%-latency-regression detection; `results/calibration/cjson-service-aa.json`). **The verified batch win phase3-cjson-002 measured under a 150 rps-target replay (20 interleaved rounds, 0 drops — the `phase4-cjson-service` ledger row records sessions=20, rate_rps=150, drop_rate=0.0; total request count is not machine-recorded): p50 latency +6.2%, 95% CI [+5.8%, +7.2%] — accepted; p99 NOT claimed (CI [0.07, 4.97], single-worker loopback tail jitter) — harness correctly rejected it.** Mechanical ROI report generator (`crates/report` bin, `just report`, SPEC §9): reads a ledger row + service-latency JSON → throughput→cores→dollars (27.5 cores / $9,621-per-year CI lower bound on a 500-core fleet) *and* latency percentiles, every figure with CI + workload + methodology inline; flags any non-accepted row. Case study: `results/phase4/`. **The roadmap is complete — the profile→hypothesize→patch→gated-verdict→ROI loop runs end to end across Rust and C, batch and service, with zero shipped false accepts across all phases.**
 

docs/src/skill.md

@@ -83,7 +83,13 @@ document. In `verify` mode, the gate order is the content:
 
 Measurement requires a Linux/POSIX host — the same requirement as the rest of
 the pipeline. On Windows (or with no checkout reachable) the skill refuses and
-says why. That is asserted, not assumed: on every pull request, CI runs the
+says why. The refusal itself is cross-OS: preflight ships as both `preflight.sh`
+(POSIX) and `preflight.ps1` (Windows PowerShell 5.1 / pwsh), so a Windows-native
+harness with no POSIX shell still gets a structured `STATUS=refuse` instead of a
+launch error it might misread as license to hand-roll a timing loop. Two
+implementations of one contract is a drift risk, so CI byte-diffs their output
+on every path (resolve, refuse, `PELTIER_HOME`) on the same host, and a Windows
+runner asserts the native refusal on every pull request. That is asserted, not assumed: on every pull request, CI runs the
 preflight's refusal path (skill copied outside any checkout) and its resolve
 paths (in-checkout and via `PELTIER_HOME` from a foreign repo), alongside a
 live plumbing run of the harness it drives.