P-01 - H04 Evidence Hash Dereference

Status: RESOLVED_BLOCKING

• reviewer_evidence_hash = signoff_binding.reviewer_evidence_id -> evidence_registry.evidence_id -> evidence_registry.artifact_sha256.
• binding_evidence_hash = signoff_binding.binding_evidence_id -> evidence_registry.evidence_id -> evidence_registry.artifact_sha256.

The FK row must resolve; missing evidence invalidates signoff. UUIDs are identities, never evidence content hashes. H04 intentionally does not hash evidence UUID separately, so equal artifact bytes with equal SHA-256 remain content-equivalent across registry UUIDs.

Required fixtures reject UUID-as-content hashing and missing evidence, and prove an artifact_sha256 mutation changes H04.