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/screens/NewGame.svelte

@@ -27,6 +27,9 @@
 
   // --- auto-match ---
   let searching = $state(false);
+  // matched guards the teardown: once a match arrives (immediately, via the match_found push, or
+  // via the fallback poll) onDestroy must not dequeue the game we just got.
+  let matched = $state(false);
   let poll: ReturnType<typeof setInterval> | null = null;
 
   function stop() {
@@ -35,6 +38,24 @@
       poll = null;
     }
   }
+  // startPoll is the matchmaking fallback used only while the live stream is down: with the stream
+  // up the match_found push drives navigation (R4). It polls lobby.poll every 2.5s.
+  function startPoll() {
+    if (poll) return;
+    poll = setInterval(async () => {
+      try {
+        const p = await gateway.lobbyPoll();
+        if (p.matched && p.game) {
+          matched = true;
+          searching = false;
+          stop();
+          navigate(`/game/${p.game.id}`);
+        }
+      } catch (e) {
+        handleError(e);
+      }
+    }, 2500);
+  }
   // cancelSearch leaves the auto-match pool on the backend too, so a cancelled quick-match
   // is actually dequeued — otherwise the account stays queued (blocking a re-queue) and the
   // reaper later substitutes a robot for a game the player abandoned (Stage 17 fix).
@@ -47,31 +68,37 @@
 
   async function find(v: Variant) {
     searching = true;
+    matched = false;
     try {
       const r = await gateway.lobbyEnqueue(v);
       if (r.matched && r.game) {
+        matched = true;
         searching = false;
         navigate(`/game/${r.game.id}`);
-        return;
       }
-      poll = setInterval(async () => {
-        try {
-          const p = await gateway.lobbyPoll();
-          if (p.matched && p.game) {
-            stop();
-            searching = false;
-            navigate(`/game/${p.game.id}`);
-          }
-        } catch (e) {
-          handleError(e);
-        }
-      }, 2500);
     } catch (e) {
       searching = false;
       handleError(e);
     }
+    // No immediate match: wait for the match_found push; the effect below polls only when the
+    // stream is down.
   }
 
+  // Poll for the match only while searching and the stream is down (the push cannot reach us);
+  // stop once the stream is back or the search ends.
+  $effect(() => {
+    if (searching && !app.streamAlive) startPoll();
+    else stop();
+  });
+  // match_found is handled globally (app.svelte navigates); end the search here too so onDestroy
+  // does not cancel the match we just received.
+  $effect(() => {
+    if (app.lastEvent?.kind === 'match_found' && searching) {
+      matched = true;
+      searching = false;
+    }
+  });
+
   // --- friend game ---
   let friends = $state<AccountRef[]>([]);
   let selected = $state<string[]>([]);
@@ -120,8 +147,9 @@
 
   onDestroy(() => {
     stop();
-    // Abandoned mid-search (navigated away without Cancel): dequeue so we don't linger.
-    if (searching) void gateway.lobbyCancel().catch(() => {});
+    // Abandoned mid-search without a match (navigated away without Cancel): dequeue so we don't
+    // linger. A received match (matched) must not be cancelled.
+    if (searching && !matched) void gateway.lobbyCancel().catch(() => {});
   });
 </script>