feat: validate immutable signal provenance manifests

[Pigbibi]

Scope

• bind shadow-signal requests to the immutable source commit and context bytes digest
• validate sibling manifest v2 before accepting a committed latest signal
• mark manifest v1 as legacy_untrusted

Safety

• no workflow, provider, AIAuditBridge, signal-generation, credential, or execution-path change
• current committed v1/stale artifact is deliberately not rewritten

Verification

• RED: stale digest, missing input digest, mutable producer ref, and v1 legacy cases failed before implementation
• PYTHONPATH=src:. python3 -m pytest -q (48 passed)
• python3 -m ruff check scripts/post_shadow_signal_request.py scripts/validate_latest_signal.py tests/test_signal_validation.py
• git diff --check

Compatibility

Existing v1 manifests fail closed as legacy_untrusted; a fresh producer-generated v2 artifact is required.