feat: add revisioned resync reducer

multiplayer-todo.md

@@ -1227,7 +1227,7 @@ the local/CI/community transport, not a silent production fallback.
 | # | Task | Acceptance |
 |---|---|---|
 | 8.39 `[D:8.3,8.14,8.17]` | Queue UI: playlist/quality, elapsed and estimated wait, proposal countdown, allocation/connect state, cancel and latency/capacity explanations | Every backend state and terminal failure has a non-stuck visible state; cancel/decline is acknowledged authoritatively |
-| 8.40 `[D:8.3,8.14]` | One authenticated revisioned WebSocket plus REST resync; resume valid queue/assignment after client restart | Missed/duplicate/out-of-order events converge and restart never creates a second ticket |
+| 8.40 `[D:8.3,8.14]` | **IN PROGRESS.** Pure Go revisioned replica reducer rejects gaps for REST resync, makes duplicate/out-of-order events idempotent, and resumes from the authoritative snapshot revision | `server/domain/sync.go` covers gap, snapshot, replay and same-revision conflict behavior; authenticated WebSocket/REST transport, client restart persistence and duplicate-ticket integration remain |
 | 8.41 `[D:7.8,8.9,8.31,8.40]` | After assignment-ready, install SDR relay ticket before connect and send match-scoped join authorisation in `hello`; retain ENet assignments locally | Production connects/reconnects/fences old generation through SDR, never before assignment-ready; allocated/direct ENet and community flows remain compatible |
 | 8.42 `[D:8.22,8.23,8.24,8.40]` | Backend-authoritative provisional/rank/tier/delta, abandon status and season countdown UI | Client performs no rating math and displays the committed revision after reconnect |
 | 8.43 `[D:8.39,8.40,8.41]` | Recovery paths for decline, expiry, startup failure, version mismatch, auth expiry, regional outage and failed reconnect | Automated UI/state tests prove every case returns to a usable queue/menu or resumes the match without a duplicate action |