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/lobby.go

@@ -14,12 +14,17 @@ import (
 
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/notify"
 )
 
 // GameCreator is the slice of the game domain the lobby needs: starting a seated
-// game. game.Service satisfies it.
+// game and reading a player's initial view of it. game.Service satisfies it.
 type GameCreator interface {
 	Create(ctx context.Context, params game.CreateParams) (game.Game, error)
+	// InitialState returns a seated player's full initial view of a started game, used
+	// to enrich the match_found / game_started events so the client renders the new game
+	// without a follow-up fetch (R4).
+	InitialState(ctx context.Context, gameID, accountID uuid.UUID) (notify.PlayerState, error)
 }
 
 // RobotProvider supplies a robot account to substitute for a missing human in