Guarantee durable settlement recovery

[amavashev]

Summary

• writes pending commit and expiry-event fallback state atomically before the first settlement request
• recovers expired reservations through POST /v1/events and replays unresolved settlement across restart with the same idempotency key
• accepts only exact, schema-valid HTTP 200 commit and HTTP 201 event responses as terminal success
• quarantines corrupt, structurally invalid, and unsupported-version journal records while continuing valid replay
• replaces collision-prone legacy journal filenames with the shared full-SHA-256 identity and safe migration
• proves lost-response, restart, rate-limit, key-rotation, concurrent-replay, heartbeat, and known-actual-boundary behavior with exact native tests
• pins CI and publishing to the reviewed protocol profile 0.3 commit

Context

Ports the durable journal and expiry-event recovery guarantees already present in other SDKs and implements the contract in runcycles/cycles-protocol#149.

Validation

• full Python 3.10 and 3.12 lint/test jobs pass
• source MyPy and Ruff checks pass
• complete 12-scenario recovery conformance matrix passes
• all latest-head PR checks pass