feat(guardian): distributed execution authority consumption

[aeonmatrixcommand-debug]

Summary

Adds distributed, persistent consumption semantics for Guardian execution authority.

Changes

• enforce single-use execution authority
• abstract authorization consumption storage
• delegate atomicity to the storage backend
• add Google Cloud Datastore implementation
• use deterministic authorization entity keys
• add bounded retry handling for Datastore transaction contention
• repair the Guardian runtime demo publisher contract
• add replay, shared-store, cross-instance, concurrency, atomicity, and Datastore tests

Validation

• Guardian suite: 71 passed
• Python compilation: PASS
• diff validation: PASS
• Datastore sequential live contract: PASS
• Datastore concurrent retry contract: PASS
• concurrent result: exactly 1 winner, 7 rejected claims, 0 errors
• live probe cleanup: PASS
• local/remote commit SHA verified

Security invariant

An execution authorization may be consumed successfully at most once across competing Guardian instances.

Datastore transaction contention is retried with a bounded retry policy. Losing claimants observe the persisted consumption marker and return false.

Notes

Existing datetime.utcnow() deprecation warnings are outside the scope of this change.