feat: add durable SQLite state and idempotency ledger

[cvsz]

Summary

Implements the first bounded slice of EP-01 Durable State and Recovery:

• add SQLiteStateStore
• WAL mode + synchronous=FULL
• atomic checkpoint updates via BEGIN IMMEDIATE
• checkpoint integrity validation on read
• durable idempotency records
• fail-closed idempotency status validation
• prevent successful side effects from being moved backward into retry states
• add deterministic unit tests for checkpoints and idempotency behavior

Acceptance evidence targeted by CI

python -m unittest discover -s src -p 'test_*.py' -v

This PR does not yet claim all of EP-01 complete. Resume API, cancellation/deadlines, PostgreSQL contract, and full state-machine recovery remain follow-up slices.

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