feat(merlin): add delivery receipts - #53

Summary

• return a versioned Merlin receipt with canonical digest, segment ID, status, and accepted/duplicate counts
• cover the receipt in the core and idempotency integration tests
• document the contract in the README and OpenAPI description

Test plan

• cargo fmt --check
• cargo test -p corpus-core --lib
• cargo test -p corpus-core --test merlin
• cargo test -p corpus-server

Rollout

The response remains replay-safe and additive. Merlin will only mark a segment delivered after validating this receipt.