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

corpora/README.md

@@ -0,0 +1,15 @@
+# corpora/
+
+Golden-replay input corpora and expected outputs, one directory per
+target. **Trust layer: read-only to the agent** (SPEC §10).
+
+Every corpus directory carries a `MANIFEST.sha256` (`sha256sum` format:
+`<digest>  <relative-path>` per line). `diff-test` verifies the manifest
+before every run and refuses to run on any mismatch — a tampered corpus
+is a stop-the-line event.
+
+Regenerate a manifest only as a deliberate human action:
+
+```sh
+cd corpora/<target> && find . -type f ! -name MANIFEST.sha256 -exec sha256sum {} + > MANIFEST.sha256
+```