fix(keyless): bind proof-cache key to full entry + propagate audit-hash error (v0.9.2)

[avrabe]

Continues the STPA-Sec keyless-verify hardening campaign from v0.9.1 (#136). Closes UCA-4 (#139) and UCA-5 (#140) from docs/security/stpa-keyless-2026-05-25.md.

⚠️ UCA-1 (#137) was scoped in but descoped — important finding

Wiring the existing Merkle inclusion-proof verifier into verify() (the original plan for this release) revealed via the "Sign & Verify Example" CI job that the verifier recomputes the wrong Merkle root for fresh production Rekor entries on the log2025-* shards (Rekor v2 / tiled-log migration):

Rekor SET verified successfully
Rekor error: Computed root hash does not match expected root.
  Computed: a3dd3a1b…  Expected: b32b6966…   (log index 1672253805)

The verifier had never been on the production path, so this latent breakage was never observed. Enabling it fail-closed would reject all legitimate keyless signatures — strictly worse than the silent gap. UCA-1 is reclassified on #137 from "wire in the existing verifier" to "fix the verifier against Rekor v2, then wire it in". UCA-3 (#138) remains deferred (clock-policy decision).

What this PR fixes

Issue	UCA	Fix
#139	UCA-4 — proof-cache hit skipped SET re-verify, trusted attacker bundle fields	CacheKey::from_entry binds every entry field (length-prefixed, unambiguous) instead of (module_hash, uuid). A hit now requires a byte-identical entry that already passed SET verification.
#140	UCA-5 — audit log recorded SHA-256 of zero bytes on serialize failure	Propagate the serialize error (? instead of .ok()), so a serialize failure aborts verify instead of writing a false artifact identity to the forensic trail.

Verification / oracles

• UCA-4 has a pure-function mechanical oracle: CacheKey::from_entry tests prove every field mutation changes the key, plus a field-boundary-ambiguity test (("ab","c") ≠ ("a","bc")).
• UCA-5: serialize call now propagates via map_err(..)?; no .ok() remains on it.
• A cold clean-room subagent verified all six original claims with file:line citations before the UCA-1 descope; the cache-binding and audit-hash claims it confirmed are exactly what ships here.
• 609 library tests pass; full workspace builds.

Falsification statement

A keyless proof-cache hit can no longer be induced by mutating the embedded Rekor entry while keeping the module bytes — any field change forces full SET re-verification. A module that fails to serialize no longer records sha256:e3b0c442…b855 in the audit log; it aborts with a non-zero exit.

Closes #139
Closes #140

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.75000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/lib/src/signature/keyless/signer.rs	0.00%	5 Missing ⚠️
src/lib/src/signature/keyless/format.rs	72.72%	3 Missing ⚠️
src/lib/src/signature/keyless/proof_cache.rs	98.43%	1 Missing ⚠️

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