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.

pkg/fbs/scrabblefb/YourTurnEvent.go

@@ -93,8 +93,20 @@ func (rcv *YourTurnEvent) ScoreLine() []byte {
 	return nil
 }
 
+func (rcv *YourTurnEvent) MoveCount() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(16))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *YourTurnEvent) MutateMoveCount(n int32) bool {
+	return rcv._tab.MutateInt32Slot(16, n)
+}
+
 func YourTurnEventStart(builder *flatbuffers.Builder) {
-	builder.StartObject(6)
+	builder.StartObject(7)
 }
 func YourTurnEventAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(gameId), 0)
@@ -114,6 +126,9 @@ func YourTurnEventAddLastWord(builder *flatbuffers.Builder, lastWord flatbuffers
 func YourTurnEventAddScoreLine(builder *flatbuffers.Builder, scoreLine flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(5, flatbuffers.UOffsetT(scoreLine), 0)
 }
+func YourTurnEventAddMoveCount(builder *flatbuffers.Builder, moveCount int32) {
+	builder.PrependInt32Slot(6, moveCount, 0)
+}
 func YourTurnEventEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }