[nosam1998]

External players can create a match, submit moves against an expected turn, retrieve their view, and replay every accepted action. State is rebuilt from the original engine version, configuration, seed, custom position, and canonical actions; mismatched transition events or completion records fail explicitly.

Moves persist atomically with optional idempotency receipts. Duplicate retries return their original response, stale writes include current retry context, and rejected moves or storage failures leave no partial events. A declared game-length cap records a replayable truncation.

Depends on #8. This is a trusted in-process service; network authorization, SQL adapters, opponent scheduling, and transport endpoints are separate units.

Validation: all GitHub Actions checks pass at d864f3c95e6b63a8f17407b44f20101929720f0c, including 21 Rust tests, fmt, clippy, dependency boundaries, cargo-deny, core API compatibility, web checks, and game benchmarks. Eight new lifecycle tests cover complete games, all action encodings, stochastic replay, viewer projections, custom starts, idempotency races, storage failure, truncation, and corrupt logs.