[#543] Guarantee commitment identity uniqueness across instances

[arisu6804]

Summary

This PR implements the collision-safe commitment identity policy requested in #543.

• namespaces commitment identities by contract instance and sequence;
• makes exact retries replay the original identity without a second record;
• rejects same-identity payload changes as fingerprint conflicts;
• rejects cross-instance collisions and malformed identities;
• adds bounded registry and migration guards for deterministic contract behavior;
• documents canonical fingerprint inputs, adapter ordering, rollout, monitoring, and rollback limitations;
• adds decision-matrix and fuzz-style regression coverage.

The change set contains 504 added lines, including policy code, tests, and operational documentation.

Validation

• git diff --cached --check
• deterministic unit and matrix coverage included for malformed IDs, collisions, replay, capacity, and migration paths.

Closes #543