feat: guarantee durable per-record migration outcomes

[shhahhussain]

Summary

• validate complete memory receipts and document batch outcomes
• isolate malformed memory records through bounded batch division
• journal one durable latest outcome per source record
• retry only unresolved retryable writes
• preserve progress from jobs created by earlier releases
• release getmnemo-migrate 0.3.0

Verification

• 43 tests passed
• typecheck, build, OpenAPI contract, and package dry-run passed

Safety

• remote retries use deterministic custom IDs or idempotency keys
• malformed successful responses fail closed
• no new dependency or backend schema change