fix: make exact M0 source retries idempotent

[Pigbibi]

Summary

• acknowledge an authenticated retry of the same immutable M0 source artifact without writing KV again
• retain rejection for distinct-source replay and ledger-time rollback
• document the delivery semantics and add Worker regression coverage

Verification

• Python M0 publisher, envelope, and ledger tests: 19 passed
• Strategy-switch Worker validation passed

Research-only M0 ingestion change. No strategy activation, runtime target, broker, order, or profile-KV behavior is changed.