R4: push enrichment — events carry a state delta, kill the last poll

Enrich the in-app live stream into a delta channel so the UI renders a move from the event without a follow-up game.state, and make the matchmaking poll a stream-down fallback.

- pkg/fbs: trailing fields on opponent_moved (move+game+bag_len), your_turn (move_count), match_found (state), game_over (game), notify (account/invitation/state), MoveResult (rack+bag_len); regenerate Go + TS.
- backend: notify owns the FB encoding (encode.go + payload.go input structs); game/lobby/social map their domain types in. emitMove builds the move delta; game.Service.InitialState feeds match_found/game_started the recipient's initial StateView; friends/invitations notify carry their account/invitation. The move-commit response (submit_play/pass/exchange/resign) returns the actor's refilled rack + bag size.
- gateway: MoveResult transcode carries rack+bag_len.
- ui: pure lib/gamedelta.ts reducer advances the per-game cache keyed on move_count (idempotent + gap-safe); app.svelte seeds the cache on match_found/game_started; Game.svelte applies the delta (commit/pass/exchange/resign drop their load()); NewGame polls only while app.streamAlive is false.
- docs: ARCHITECTURE §10, FUNCTIONAL(+ru), backend/gateway/ui READMEs; PRERELEASE R4 marked done + Refinements.

ui/src/lib/mock/client.ts

@@ -235,7 +235,7 @@ export class MockGateway implements GatewayClient {
     g.view.toMove = (seat + 1) % g.view.players;
     this.drafts.delete(gameId);
     this.scheduleOpponentReply(gameId);
-    return { move: structuredClone(move), game: structuredClone(g.view) };
+    return { move: structuredClone(move), game: structuredClone(g.view), rack: structuredClone(g.rack), bagLen: g.bagLen };
   }
 
   private async simpleAction(
@@ -276,7 +276,7 @@ export class MockGateway implements GatewayClient {
       g.view.toMove = (seat + 1) % g.view.players;
       this.scheduleOpponentReply(gameId);
     }
-    return { move: structuredClone(move), game: structuredClone(g.view) };
+    return { move: structuredClone(move), game: structuredClone(g.view), rack: structuredClone(g.rack), bagLen: g.bagLen };
   }
 
   pass(gameId: string): Promise<MoveResult> {
@@ -546,8 +546,8 @@ export class MockGateway implements GatewayClient {
       g.view.seats[opp].score = move.total;
       g.view.moveCount += 1;
       g.view.toMove = this.mySeat(g);
-      this.emit({ kind: 'opponent_moved', gameId, seat: opp, action: 'play', score: 6, total: move.total });
-      this.emit({ kind: 'your_turn', gameId, deadlineUnix: Math.floor(Date.now() / 1000) + 86400 });
+      this.emit({ kind: 'opponent_moved', gameId, move: structuredClone(move), game: structuredClone(g.view), bagLen: g.bagLen });
+      this.emit({ kind: 'your_turn', gameId, deadlineUnix: Math.floor(Date.now() / 1000) + 86400, moveCount: g.view.moveCount });
     }, 1600);
   }