Stage360 External Timestamp Proof Binding Layer

Decision:
pending_timestamp_proof

What Stage360 adds:
- Reads Stage359 public key fingerprint result
- Binds Stage359 result_sha256 as Stage360 previous_hash
- Uses Stage359 public_key_fingerprint as timestamp target input
- Generates timestamp_target_sha256
- Records OpenTimestamps proof metadata
- Records RFC3161 proof metadata
- Returns pending_timestamp_proof when real proof is not present
- Blocks fake verified timestamp claims
- Does not claim real OpenTimestamps verification
- Does not claim real RFC3161 verification

Previous hash from Stage359:
a081c57c22eebf76384a2c8d4a9bc7cba13a7013705095c97fd1ec4ceaee6a95

Stage360 entry hash:
23591f8724a6b7ea0c7b74d6a1c26e4568277223250df64553ce65d286dcc2d7

Timestamp target SHA256:
052c8f0283110e405443d56f2396c52a8486e7a70a489f831af107dad73ab1b5

Proof status:
pending_timestamp_proof

Generated files:
- docs/timestamp-proof/stage360_external_timestamp_proof_policy.json
- docs/timestamp-proof/stage360_external_timestamp_proof_result.json
- docs/timestamp-proof/stage360_external_timestamp_proof_summary.txt

Safety boundary:
- No private keys
- No raw secrets
- No real OTS verification claim
- No real RFC3161 verification claim
- No external timestamp verified claim
- Timestamp proof binding metadata only