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/codec.ts

@@ -322,12 +322,12 @@ export function decodeProfile(buf: Uint8Array): Profile {
   };
 }
 
-export function decodeStateView(buf: Uint8Array): StateView {
-  const v = fb.StateView.getRootAsStateView(new ByteBuffer(buf));
+// decodeStateViewTable projects a StateView table (a root or one nested in an event) to the
+// model. It caches the alphabet when present (a per-variant cache miss) and decodes the index
+// rack to display letters with it (Stage 13).
+function decodeStateViewTable(v: fb.StateView): StateView {
   const g = v.game();
   const variant = (g ? s(g.variant()) : 'scrabble_en') as Variant;
-  // Cache the alphabet table when the server included it (a per-variant cache miss), then
-  // decode the index rack to display letters with it (Stage 13).
   if (v.alphabetLength() > 0) {
     const entries: AlphabetEntryWire[] = [];
     for (let i = 0; i < v.alphabetLength(); i++) {
@@ -347,11 +347,24 @@ export function decodeStateView(buf: Uint8Array): StateView {
   };
 }
 
+export function decodeStateView(buf: Uint8Array): StateView {
+  return decodeStateViewTable(fb.StateView.getRootAsStateView(new ByteBuffer(buf)));
+}
+
 export function decodeMoveResult(buf: Uint8Array): MoveResult {
   const r = fb.MoveResult.getRootAsMoveResult(new ByteBuffer(buf));
   const m = r.move();
   const g = r.game();
-  return { move: m ? decodeMove(m) : emptyMove(), game: g ? decodeGameView(g) : emptyGame() };
+  // The actor's refilled rack rides back as alphabet indices (R4); decode it with the game's variant.
+  const variant = (g ? s(g.variant()) : 'scrabble_en') as Variant;
+  const rack: string[] = [];
+  for (let i = 0; i < r.rackLength(); i++) rack.push(letterForIndex(variant, r.rack(i) ?? 0));
+  return {
+    move: m ? decodeMove(m) : emptyMove(),
+    game: g ? decodeGameView(g) : emptyGame(),
+    rack,
+    bagLen: r.bagLen(),
+  };
 }
 
 export function decodeHintResult(buf: Uint8Array): HintResult {
@@ -424,11 +437,30 @@ export function decodeEvent(kind: string, payload: Uint8Array): PushEvent | null
   switch (kind) {
     case 'your_turn': {
       const e = fb.YourTurnEvent.getRootAsYourTurnEvent(bb);
-      return { kind: 'your_turn', gameId: s(e.gameId()), deadlineUnix: Number(e.deadlineUnix()) };
+      return { kind: 'your_turn', gameId: s(e.gameId()), deadlineUnix: Number(e.deadlineUnix()), moveCount: e.moveCount() };
     }
     case 'opponent_moved': {
       const e = fb.OpponentMovedEvent.getRootAsOpponentMovedEvent(bb);
-      return { kind: 'opponent_moved', gameId: s(e.gameId()), seat: e.seat(), action: s(e.action()), score: e.score(), total: e.total() };
+      const m = e.move();
+      const g = e.game();
+      return {
+        kind: 'opponent_moved',
+        gameId: s(e.gameId()),
+        move: m ? decodeMove(m) : undefined,
+        game: g ? decodeGameView(g) : undefined,
+        bagLen: e.bagLen(),
+      };
+    }
+    case 'game_over': {
+      const e = fb.GameOverEvent.getRootAsGameOverEvent(bb);
+      const g = e.game();
+      return {
+        kind: 'game_over',
+        gameId: s(e.gameId()),
+        result: s(e.result()),
+        scoreLine: s(e.scoreLine()),
+        game: g ? decodeGameView(g) : undefined,
+      };
     }
     case 'chat_message':
       return { kind: 'chat_message', message: decodeChatMsg(fb.ChatMessage.getRootAsChatMessage(bb)) };
@@ -438,11 +470,21 @@ export function decodeEvent(kind: string, payload: Uint8Array): PushEvent | null
     }
     case 'match_found': {
       const e = fb.MatchFoundEvent.getRootAsMatchFoundEvent(bb);
-      return { kind: 'match_found', gameId: s(e.gameId()) };
+      const st = e.state();
+      return { kind: 'match_found', gameId: s(e.gameId()), state: st ? decodeStateViewTable(st) : undefined };
     }
     case 'notify': {
       const e = fb.NotificationEvent.getRootAsNotificationEvent(bb);
-      return { kind: 'notify', sub: s(e.kind()) };
+      const acc = e.account();
+      const inv = e.invitation();
+      const st = e.state();
+      return {
+        kind: 'notify',
+        sub: s(e.kind()),
+        account: acc ? decodeAccountRef(acc) : undefined,
+        invitation: inv ? decodeInvitationTable(inv) : undefined,
+        state: st ? decodeStateViewTable(st) : undefined,
+      };
     }
     case 'heartbeat':
       return { kind: 'heartbeat' };