feat(evidence): record model disclosure audit metadata

[seonghobae]

Partial implementation of #10, stacked on PR #45.

Buyer/security gap

Protected-main data governance requires credential-free sensitive-access evidence to retain model/provider/region/retention metadata when protected data is disclosed to a model. The current evidence receipt records the sensitive-data decision and handle lifecycle but has no bounded model-route audit metadata, so an operator cannot reconstruct which reviewed model route governed an AI-involved disclosure without relying on external logs.

Dependency

This Draft is stacked on unchanged exact PR #45 head 0f07fea031090c72a448fd9501b49d4dd7568419. That prerequisite owns the shared sensitive-evidence identifier validation surface; this lane must remain Draft while #45 is active.

TDD / RCA sequence

1. Test-only head ca57239960af4abd09aabc90ddb36c9caefc2798 introduced the intended credential-free model-disclosure evidence contract, but CI run 31521173221 stopped first at canonical rustfmt. That setup failure is not counted as the intended RED.
2. Formatting-only head 3c4ae22e1f16527b4ad63c9fe58095142e763a25 reached the real production boundary. CI run 31521597109, Rust contracts job 93879681560, passed repository contracts and canonical formatting, then cargo check --locked --workspace --all-targets failed with E0432 because SensitiveModelDisclosureEvidence and SensitiveModelDisclosureEvidenceInput did not exist. This is the valid exact-head RED.
3. Production commit b1c8a79660a776e25337c65c598afb4be1e105a9 adds the narrow immutable evidence value, and af8af758a7d4873e2fa28a662f22f9296aeb8fe6 exposes it through the evidence crate root.
4. Exact production head af8af758a7d4873e2fa28a662f22f9296aeb8fe6 passed CI run 31522004452: Rust contracts job 93881137254 and Production coverage job 93881137057 both succeeded, including repository contracts, formatting, locked workspace/all-target check, full tests, strict Clippy, rustdoc, and exact owned production function/line/region/branch coverage.
5. Current exact head 3a33f83af7398038a2581e2e132fabf7183b17af adds only the truthful Unreleased changelog entry and has now passed fresh current-head CI run 31522457974. CodeRabbit's exact-head commit status is also success; GitHub reports the PR mergeable, and no formal reviews or inline review threads are currently returned. No predecessor-head result is promoted as current proof.

Implemented contract

SensitiveModelDisclosureEvidence is an immutable credential-free value object linked to the originating sensitive-data request_id and decision_id. It records only bounded reviewed identifiers for provider, model, processing region, retention policy, training policy, subprocessor policy, and export policy. Every identifier uses the existing sensitive-evidence validation contract; malformed, punctuation-only, whitespace-bearing, slash-bearing, or oversized identifiers fail closed as SensitiveEvidenceError::InvalidIdentifier.

The integration regressions exercise every public getter, every identifier position, the shared maximum identifier bound, and the absence of protected-value/provider-credential sentinel material from debug evidence.

Truth boundary

This receipt is audit metadata only. It does not authorize disclosure, authenticate a workload/provider, invoke a model, prove runtime route/region/retention/training/subprocessor/export behavior, carry prompts or outputs, store protected values or provider credentials, implement durable/tamper-evident audit sequencing, or close #10. Runtime/provider enforcement and protected-value disclosure remain separate trusted broker/orchestrator responsibilities.

Keep this PR Draft while #45 remains active. Any prerequisite or current-head movement requires fresh exact-head/live-base validation.

[coderabbitai]

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