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/scrabble.fbs

@@ -159,10 +159,16 @@ table SubmitPlayRequest {
   tiles:[PlayTile];
 }
 
-// MoveResult is the outcome of a committed move: the move and the post-move game.
+// MoveResult is the outcome of a committed move: the move and the post-move game. rack and
+// bag_len carry the actor's own post-move private state — their refilled rack (alphabet indices,
+// Stage 13; a blank is the sentinel index 255) and the bag size after drawing — so the mover
+// renders the next state straight from this response without a follow-up game.state (R4; added
+// trailing — backward-compatible).
 table MoveResult {
   move:MoveRecord;
   game:GameView;
+  rack:[ubyte];
+  bag_len:int;
 }
 
 // StateRequest asks for the requesting player's view of a game. include_alphabet asks the
@@ -477,6 +483,8 @@ table GcgExport {
 // move kind ("play"/"pass"/"exchange"/...), last_word is the main word of a scoring play (empty
 // otherwise), and score_line is the recipient-first running score (e.g. "120:95:80"). They are
 // appended (FlatBuffers-optional), so an older reader that only needs game_id/deadline is unaffected.
+// move_count is the post-move count (matching the opponent_moved GameView): the client uses it to
+// tell whether its cached game already reflects the move, falling back to a refetch on a gap (R4).
 table YourTurnEvent {
   game_id:string;
   deadline_unix:long;
@@ -484,25 +492,35 @@ table YourTurnEvent {
   last_action:string;
   last_word:string;
   score_line:string;
+  move_count:int;
 }
 
 // GameOverEvent signals that a game the recipient is seated in has finished, driving the
 // out-of-app "game over" push (Stage 17). result is the outcome from the recipient's own
-// perspective ("won"/"lost"/"draw"); score_line is the recipient-first final score.
+// perspective ("won"/"lost"/"draw"); score_line is the recipient-first final score. game is the
+// final post-game summary (adjusted scores after rack penalties + the winner flag), so the client
+// settles the finished game from the event without a refetch (R4; added trailing).
 table GameOverEvent {
   game_id:string;
   result:string;
   score_line:string;
+  game:GameView;
 }
 
-// OpponentMovedEvent summarises a move another seat just committed; the client
-// refetches the full state.
+// OpponentMovedEvent carries a move another seat just committed as a delta the client applies to
+// its cached game without a refetch (R4): move is the decoded play/pass/exchange (the same record
+// game.history returns), game is the post-move summary (per-seat scores, to_move, move_count,
+// status) and bag_len is the bag size after the draw. The leading seat/action/score/total scalars
+// are the pre-R4 summary, now redundant with move/game and kept only for wire back-compat.
 table OpponentMovedEvent {
   game_id:string;
   seat:int;
   action:string;
   score:int;
   total:int;
+  move:MoveRecord;
+  game:GameView;
+  bag_len:int;
 }
 
 // NudgeEvent signals that a player nudged the recipient.
@@ -511,17 +529,27 @@ table NudgeEvent {
   from_user_id:string;
 }
 
-// MatchFoundEvent signals that an auto-match pairing (or robot substitution)
-// started a game the recipient is seated in.
+// MatchFoundEvent signals that an auto-match pairing (or robot substitution) started a game the
+// recipient is seated in. state is the recipient's full initial view of the new game (empty board,
+// dealt rack), so the client navigates straight in from the event with no follow-up fetch (R4;
+// added trailing — an older reader still reads just game_id).
 table MatchFoundEvent {
   game_id:string;
+  state:StateView;
 }
 
 // NotificationEvent is a lightweight "something changed, re-poll" signal that
 // drives the lobby badge (incoming friend requests, invitations). kind is a sub-
 // discriminator ("friend_request", "friend_added", "friend_declined", "invitation",
 // "game_started"); the client re-fetches its lobby counters (and, for a requester
-// watching a game, its friend state) on any of them.
+// watching a game, its friend state) on any of them. To let the client update its lobby without a
+// follow-up fetch (R4), each event also carries the payload its kind changed: account for the
+// friend_* kinds (the requester/friend), invitation for "invitation" (the new invitation) and
+// state for "game_started" (the started game's initial view, like match_found). Only the field
+// matching kind is set (all added trailing — backward-compatible).
 table NotificationEvent {
   kind:string;
+  account:AccountRef;
+  invitation:Invitation;
+  state:StateView;
 }