Implement AUDIT-WP-0007 hash-chain integrity.

Accept now extends a single-schema chain. Verify walks it; a rewritten
payload_hash is a break. Tamper evidence is that detector plus an
external chain-head attestation, not WORM.

registry/capabilities/capability.audit.event-retain.md

@@ -23,7 +23,6 @@ external_evidence:
       - recovery cited to the live platform data.backup provision
     broken_expectations:
       - data.archive sink not provided
-      - tamper evidence not implemented
     out_of_scope_expectations:
       - application business audit semantics ownership
       - booked-cost origination
@@ -33,7 +32,6 @@ external_evidence:
     basis: failure_matrix_and_restore_walk
     known_reliability_risks:
       - single replica
-      - integrity_verification hook unmet
 
 discovery:
   intent: >