fix: make transfer lifecycle transitions concurrency-safe

[tokenjunkielabs]

Fixes #133.

This makes terminal transfer mutations concurrency-safe within the backend's current store boundary and gives clients an explicit optimistic-concurrency contract.

What changed

• transfers now carry a monotonically increasing resource version;
• single-transfer responses expose that version as a strong ETag;
• claim/cancel require If-Match plus the existing Idempotency-Key convention;
• lifecycle mutation checks expected version + allowed transition before provider work, re-checks immediately before commit, and returns explicit 409 conflict details on stale state;
• terminal claim/cancel operations have a separate actor-scoped replay ledger so retries replay the first terminal result;
• mock Stellar settlement/claimable-balance adapters accept stable operation keys and return the same provider artifact on retry;
• provider failure occurs before terminal state commit and releases the reservation, preserving safe retry;
• archive/unarchive advance the same resource version so a stale terminal mutation cannot silently commit after another mutation;
• added focused regression source for race, duplicate callback, worker-module reload, rollback/retry, and one-terminal-outcome behavior;
• documented the HTTP/storage concurrency contract and the required durable CAS boundary for a future database-backed store.

Claimant fork head: 373c6fb23d130df6f754ba7ad850c105e9ab0d94.

[tokenjunkielabs]

Maintainer review requested for claimant-owned bounty carrier fixing #133. This PR is OPEN and GitHub currently reports it mergeable at exact head 373c6fb23d130df6f754ba7ad850c105e9ab0d94. Claimant-side payout setup is complete. Please merge if the concurrency-safe transfer lifecycle implementation meets #133, or leave concrete change requests here; on acceptance, please complete the bounty award/payment path tied to the issue.