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.

backend/internal/lobby/matchmaker.go

@@ -75,11 +75,19 @@ func (m *Matchmaker) SetNotifier(p notify.Publisher) {
 
 // emitMatchFound pushes match_found to every seat of a freshly started game.
 // Emitting to a robot seat is harmless (no client subscription exists for it).
-func (m *Matchmaker) emitMatchFound(g game.Game) {
+func (m *Matchmaker) emitMatchFound(ctx context.Context, g game.Game) {
 	lang := g.Variant.Language() // route the push by the game's language, not the recipient's bot (Stage 17)
 	intents := make([]notify.Intent, 0, len(g.Seats))
 	for _, s := range g.Seats {
-		mf := notify.MatchFound(s.AccountID, g.ID)
+		state, err := m.games.InitialState(ctx, g.ID, s.AccountID)
+		if err != nil {
+			// A waiter still discovers the game through Poll (the ws-down fallback), so skip the
+			// enriched push for this seat rather than failing the match.
+			m.log.Warn("match_found initial state",
+				zap.String("game", g.ID.String()), zap.String("account", s.AccountID.String()), zap.Error(err))
+			continue
+		}
+		mf := notify.MatchFound(s.AccountID, g.ID, state)
 		mf.Language = lang
 		intents = append(intents, mf)
 	}
@@ -128,7 +136,7 @@ func (m *Matchmaker) Enqueue(ctx context.Context, accountID uuid.UUID, variant e
 	m.mu.Lock()
 	m.results[opponent] = g
 	m.mu.Unlock()
-	m.emitMatchFound(g)
+	m.emitMatchFound(ctx, g)
 	return EnqueueResult{Matched: true, Game: g}, nil
 }
 
@@ -227,7 +235,7 @@ func (m *Matchmaker) Reap(ctx context.Context, now time.Time) {
 		m.mu.Lock()
 		m.results[s.human] = g
 		m.mu.Unlock()
-		m.emitMatchFound(g)
+		m.emitMatchFound(ctx, g)
 	}
 }