fix: enforce fail-closed Rekor verification and close STPA-Sec coverage gaps

[avrabe]

Summary

• Security fix (AS-6): KeylessVerifier::verify() had a fail-open path where Rekor verification was silently skipped when the entry UUID was empty or "skipped". Now returns an error, enforcing DD-2 (fail-closed). This was discovered by the STPA-Sec analysis in PR feat: add Rivet STPA-Sec traceability and convert security docs #50.
• Coverage: 91% → 98.4% — closed 22 of 27 traceability warnings

Security fix detail

Before (fail-open, line 441-448 of signer.rs):

if !keyless_sig.rekor_entry.uuid.is_empty() && keyless_sig.rekor_entry.uuid != "skipped" {
    verifier.verify_set(&keyless_sig.rekor_entry)?;
} else {
    log::warn!("Rekor verification skipped (no Rekor entry)");  // ← silently continues
}

After (fail-closed):

if keyless_sig.rekor_entry.uuid.is_empty() || keyless_sig.rekor_entry.uuid == "skipped" {
    return Err(WSError::RekorError(
        "Rekor transparency log entry is required for keyless verification".into(),
    ));
}
verifier.verify_set(&keyless_sig.rekor_entry)?;

STPA-Sec findings summary

Finding	Severity	Status
AS-6: Rekor fail-open	High	Fixed in this PR
UCA-12: OIDC issuer not validated on signing side	Medium	Tracked, needs expected_issuer field
AS-9: Trust bundle caching	Under investigation	Agent still analyzing

Coverage improvements

• Added protects links from all 11 system constraints → security properties
• Added AS-11, AS-12 attack scenarios for UCA-1 and UCA-12
• Added CG-9 through CG-13 for 7 threat scenarios missing goals
• Added CD-10 through CD-12 and CV-9 through CV-11 for CR-9/10/11
• Added SP-9 (module hash integrity), SP-10 (cert pin integrity)

Test plan

• cargo test — all tests pass (551 passed)
• rivet validate — PASS (5 warnings remaining)
• rivet coverage — 98.4% weighted traceability
• CI: all checks should pass

🤖 Generated with Claude Code