Bootstrap hotpath from engineering spec v0.1

Commit the spec and scaffold the system it describes:

- SPEC.md: the full v0.1 engineering spec
- crates/: trust-layer Rust workspace, unit-tested
  - bench-runner: interleaved ABAB scheduling with discarded warm-ups,
    deterministic bootstrap CIs over the ratio of medians,
    CI-lower-bound accept rule, env fingerprinting, and a CLI with
    compare + A/A self-test modes (A/A verified passing end-to-end)
  - diff-test: gate outcome model, sha256 corpus pin verifier that
    refuses on mismatch, per-target equivalence policy parser
    (byte-identical default, explicit fp-tolerance opt-in)
  - ledger: append-only SQLite attempt schema with UPDATE/DELETE
    refused by triggers, anti-double-attempt query
  - report: cores-saved and dollars/yr ROI math over speedup CIs,
    pricing.toml inputs
- playbook/: classes 1-7 with preconditions, procedure, verification
  notes, and known failure modes per class
- agent/: Python package skeleton — six-tool contract and prompting
  spine, harness IPC deferred to Phase 1
- config/: accept.toml (2% threshold, 30 runs/side), pricing.toml
- targets/, corpora/, results/calibration/: layout and policy docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

playbook/README.md

@@ -0,0 +1,25 @@
+# Optimization playbook v0
+
+Classes are tried in **strict order** (SPEC §4) — cheapest first. Classes
+1–2 are the "your 30% was a linker flag" tier and are always exhausted
+before any source change. Each class documents:
+
+- **Preconditions** — the profile signature that suggests it applies
+- **Procedure** — what to actually try, cheapest variant first
+- **Verification notes** — gate interactions specific to this class
+- **Known failure modes** — where this class produces fake wins or real bugs
+
+| # | Class | Touches source? | Human review? |
+|---|-------|-----------------|---------------|
+| 1 | [Build configuration](01-build-config.md) | no | only PGO/BOLT pipeline changes |
+| 2 | [Allocator swap](02-allocator.md) | link line only | no |
+| 3 | [Allocation churn](03-allocation-churn.md) | yes | no |
+| 4 | [Data layout](04-data-layout.md) | yes | no |
+| 5 | [Algorithmic](05-algorithmic.md) | yes | no |
+| 6 | [SIMD](06-simd.md) | yes | FP-flag changes: always |
+| 7 | [Concurrency](07-concurrency.md) | yes | **always** |
+
+The agent's prompting spine (SPEC §3.5): state the hypothesis before
+patching; prefer the cheapest untried class; max two iterations per
+rejected hypothesis; a rejection with a clean ledger row is a successful
+outcome.