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/app.svelte.ts

@@ -25,7 +25,7 @@ import { parseStartParam } from './deeplink';
 import { clearSession, loadPrefs, loadSession, saveSession, savePrefs } from './session';
 import { connection, reportOffline, reportOnline, resetConnection } from './connection.svelte';
 import { isConnectionCode } from './retry';
-import { clearGameCache } from './gamecache';
+import { clearGameCache, setCachedGame } from './gamecache';
 import { clearLobby } from './lobbycache';
 import type { BoardLabelMode } from './boardlabels';
 
@@ -36,6 +36,8 @@ export interface Toast {
 
 export const app = $state<{
   ready: boolean;
+  /** Whether the live-event stream is connected; drives the matchmaking poll fallback (R4). */
+  streamAlive: boolean;
   session: Session | null;
   profile: Profile | null;
   toast: Toast | null;
@@ -53,6 +55,7 @@ export const app = $state<{
   chatUnread: Record<string, number>;
 }>({
   ready: false,
+  streamAlive: false,
   session: null,
   profile: null,
   toast: null,
@@ -68,7 +71,6 @@ export const app = $state<{
 });
 
 let unsubscribeStream: (() => void) | null = null;
-let streamAlive = false;
 let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
 let toastTimer: ReturnType<typeof setTimeout> | null = null;
 
@@ -100,7 +102,7 @@ function goForeground(): void {
   backgrounded = false;
   foregroundedAt = Date.now();
   if (!app.session) return;
-  if (!streamAlive) openStream(); // silently re-establish a stream dropped while away
+  if (!app.streamAlive) openStream(); // silently re-establish a stream dropped while away
   void refreshNotifications();
 }
 
@@ -131,7 +133,7 @@ export function handleError(err: unknown): void {
 
 function openStream(): void {
   closeStream();
-  streamAlive = true;
+  app.streamAlive = true;
   unsubscribeStream = gateway.subscribe(
     (e) => {
       reportOnline(); // a delivered event proves the gateway is reachable
@@ -151,13 +153,19 @@ function openStream(): void {
       } else if (e.kind === 'your_turn') {
         showToast(t('game.yourTurn'), 'info');
       } else if (e.kind === 'match_found') {
+        // Seed the cache from the event's initial state so the game renders instantly on arrival,
+        // then navigate (R4).
+        if (e.state) setCachedGame(e.state.game.id, e.state, []);
         navigate(`/game/${e.gameId}`);
       } else if (e.kind === 'notify') {
+        // A started invited game seeds its cache so opening it is instant; the lobby badge stays
+        // on the authoritative refresh (R4).
+        if (e.sub === 'game_started' && e.state) setCachedGame(e.state.game.id, e.state, []);
         void refreshNotifications();
       }
     },
     () => {
-      streamAlive = false;
+      app.streamAlive = false;
       // A background suspend drops the single-shot stream. Keep the indicator hidden while
       // backgrounded or just-resumed (bannerSuppressed); otherwise show "Connecting…" (the
       // reachability watcher and this scheduled retry recover it). Always schedule a retry.
@@ -173,7 +181,7 @@ function scheduleReconnect(): void {
   if (reconnectTimer || !app.session) return;
   reconnectTimer = setTimeout(() => {
     reconnectTimer = null;
-    if (app.session && !streamAlive && !backgrounded && !documentHidden()) openStream();
+    if (app.session && !app.streamAlive && !backgrounded && !documentHidden()) openStream();
   }, 4000);
 }
 
@@ -205,7 +213,7 @@ function closeStream(): void {
   }
   unsubscribeStream?.();
   unsubscribeStream = null;
-  streamAlive = false;
+  app.streamAlive = false;
 }
 
 async function adoptSession(s: Session): Promise<void> {