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.

gateway/internal/backendclient/api.go

@@ -106,10 +106,13 @@ type GameResp struct {
 	Seats            []SeatResp `json:"seats"`
 }
 
-// MoveResultResp is the outcome of a committed move.
+// MoveResultResp is the outcome of a committed move. Rack carries the actor's refilled rack as
+// wire alphabet indices and BagLen the bag size after the draw (R4).
 type MoveResultResp struct {
-	Move MoveRecordResp `json:"move"`
-	Game GameResp       `json:"game"`
+	Move   MoveRecordResp `json:"move"`
+	Game   GameResp       `json:"game"`
+	Rack   []int          `json:"rack"`
+	BagLen int            `json:"bag_len"`
 }
 
 // AlphabetEntryJSON is one letter of a variant's alphabet (its index, concrete letter and

gateway/internal/transcode/encode.go

@@ -123,9 +123,16 @@ func encodeMoveResult(r backendclient.MoveResultResp) []byte {
 	b := flatbuffers.NewBuilder(512)
 	move := buildMoveRecord(b, r.Move)
 	game := buildGameView(b, r.Game)
+	rackBytes := make([]byte, len(r.Rack))
+	for i, v := range r.Rack {
+		rackBytes[i] = byte(v)
+	}
+	rack := b.CreateByteVector(rackBytes)
 	fb.MoveResultStart(b)
 	fb.MoveResultAddMove(b, move)
 	fb.MoveResultAddGame(b, game)
+	fb.MoveResultAddRack(b, rack)
+	fb.MoveResultAddBagLen(b, int32(r.BagLen))
 	b.Finish(fb.MoveResultEnd(b))
 	return b.FinishedBytes()
 }