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.

audit_core/interface.py

@@ -69,6 +69,7 @@ class RetentionPolicy:
             "status": "ok",
             "custody_class": self.custody_class,
             "durable": self.durable,
+            "tamper_evidence": self.tamper_evidence,
         }
         if self.recoverable_days is not None or self.recoverable_source:
             payload["recoverable_days"] = self.recoverable_days