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/server/handlers_game.go

@@ -458,7 +458,11 @@ func (s *Server) userGame(c *gin.Context) (uuid.UUID, uuid.UUID, bool) {
 
 // writeMoveResult emits a committed move with seat display names filled in.
 func (s *Server) writeMoveResult(c *gin.Context, res game.MoveResult) {
-	dto := moveResultDTOFrom(res)
+	dto, err := moveResultDTOFrom(res)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
 	s.fillSeatNames(c.Request.Context(), &dto.Game, map[string]string{})
 	c.JSON(http.StatusOK, dto)
 }