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/notify/payload.go

@@ -0,0 +1,89 @@
+package notify
+
+// The structs below are the wire-agnostic inputs the domain services hand to the
+// enriched event constructors. Keeping them here — rather than importing the wire
+// schema into game/lobby/social — preserves the package boundary: notify owns the
+// FlatBuffers encoding, while the domain only fills in already-resolved values (seat
+// display names, alphabet-index racks). Each mirrors the matching scrabblefb table.
+
+// SeatStanding is one seat's public standing inside a GameSummary (mirrors
+// scrabblefb.SeatView).
+type SeatStanding struct {
+	Seat        int
+	AccountID   string
+	DisplayName string
+	Score       int
+	HintsUsed   int
+	IsWinner    bool
+}
+
+// GameSummary is the shared, non-private game state embedded in enriched events
+// (mirrors scrabblefb.GameView). LastActivityUnix is the lobby sort key: the current
+// turn's start for an active game, the finish time once finished.
+type GameSummary struct {
+	ID               string
+	Variant          string
+	DictVersion      string
+	Status           string
+	Players          int
+	ToMove           int
+	TurnTimeoutSecs  int
+	MoveCount        int
+	EndReason        string
+	Seats            []SeatStanding
+	LastActivityUnix int64
+}
+
+// AlphabetLetter is one variant alphabet entry (a display-only row) embedded in an
+// initial PlayerState so a client seeing a variant for the first time can render its
+// rack (mirrors scrabblefb.AlphabetEntry).
+type AlphabetLetter struct {
+	Index  int
+	Letter string
+	Value  int
+}
+
+// PlayerState is a player's full initial view of a game — the shared summary plus
+// their private rack and budgets (mirrors scrabblefb.StateView). Rack carries wire
+// alphabet indices (a blank is the sentinel index 255). Alphabet is set only when the
+// recipient may not have cached the variant yet (match_found / game_started).
+type PlayerState struct {
+	Game           GameSummary
+	Seat           int
+	Rack           []int
+	BagLen         int
+	HintsRemaining int
+	Alphabet       []AlphabetLetter
+}
+
+// AccountRef is a referenced account with its display name resolved (mirrors
+// scrabblefb.AccountRef).
+type AccountRef struct {
+	AccountID   string
+	DisplayName string
+}
+
+// InvitationInvitee is one invited player's seat and response inside an
+// InvitationSummary (mirrors scrabblefb.InvitationInvitee).
+type InvitationInvitee struct {
+	AccountID   string
+	DisplayName string
+	Seat        int
+	Response    string
+}
+
+// InvitationSummary is a friend-game invitation carried by the NotifyInvitation event so
+// the client adds it to its lobby list without a refetch (mirrors scrabblefb.Invitation).
+type InvitationSummary struct {
+	ID              string
+	Inviter         AccountRef
+	Invitees        []InvitationInvitee
+	Variant         string
+	TurnTimeoutSecs int
+	HintsAllowed    bool
+	HintsPerPlayer  int
+	DropoutTiles    string
+	Status          string
+	GameID          string
+	ExpiresAtUnix   int64
+}