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/model.ts

@@ -70,6 +70,9 @@ export interface StateView {
 export interface MoveResult {
   move: MoveRecord;
   game: GameView;
+  /** The actor's refilled rack after the move (R4), so the mover renders the next state without a refetch. */
+  rack: string[];
+  bagLen: number;
 }
 
 export interface HintResult {
@@ -223,12 +226,19 @@ export interface GameList {
   games: GameView[];
 }
 
-/** A live event delivered over the Subscribe stream. */
+/**
+ * A live event delivered over the Subscribe stream. The game events carry the move as a
+ * delta — move plus the post-move summary (and the bag size) — the client applies to its
+ * cached game without a refetch; match_found / game_started carry the recipient's initial
+ * StateView; notify carries the changed lobby payload (R4). The enriched fields are optional
+ * so a client falls back to a refetch when a payload is absent (a gap, or a pre-R4 peer).
+ */
 export type PushEvent =
-  | { kind: 'your_turn'; gameId: string; deadlineUnix: number }
-  | { kind: 'opponent_moved'; gameId: string; seat: number; action: string; score: number; total: number }
+  | { kind: 'your_turn'; gameId: string; deadlineUnix: number; moveCount: number }
+  | { kind: 'opponent_moved'; gameId: string; move?: MoveRecord; game?: GameView; bagLen: number }
+  | { kind: 'game_over'; gameId: string; result: string; scoreLine: string; game?: GameView }
   | { kind: 'chat_message'; message: ChatMessage }
   | { kind: 'nudge'; gameId: string; fromUserId: string }
-  | { kind: 'match_found'; gameId: string }
-  | { kind: 'notify'; sub: string }
+  | { kind: 'match_found'; gameId: string; state?: StateView }
+  | { kind: 'notify'; sub: string; account?: AccountRef; invitation?: Invitation; state?: StateView }
   | { kind: 'heartbeat' };