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/notify_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/google/uuid"
 
+	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/notify"
 	fb "scrabble/pkg/fbs/scrabblefb"
 )
@@ -61,7 +62,7 @@ func TestNopPublisherDiscards(t *testing.T) {
 
 func TestYourTurnPayloadRoundTrips(t *testing.T) {
 	uid, gid := uuid.New(), uuid.New()
-	in := notify.YourTurn(uid, gid, time.Unix(1717000000, 0), "Ann", "play", "STOOL", "120:95")
+	in := notify.YourTurn(uid, gid, time.Unix(1717000000, 0), "Ann", "play", "STOOL", "120:95", 7)
 	if in.UserID != uid || in.Kind != notify.KindYourTurn || in.EventID == "" {
 		t.Fatalf("intent metadata wrong: %+v", in)
 	}
@@ -72,6 +73,9 @@ func TestYourTurnPayloadRoundTrips(t *testing.T) {
 	if got := ev.DeadlineUnix(); got != 1717000000 {
 		t.Fatalf("deadline = %d, want 1717000000", got)
 	}
+	if got := ev.MoveCount(); got != 7 {
+		t.Fatalf("move_count = %d, want 7", got)
+	}
 	if string(ev.OpponentName()) != "Ann" || string(ev.LastAction()) != "play" ||
 		string(ev.LastWord()) != "STOOL" || string(ev.ScoreLine()) != "120:95" {
 		t.Fatalf("enriched fields wrong: name=%q action=%q word=%q score=%q",
@@ -81,7 +85,8 @@ func TestYourTurnPayloadRoundTrips(t *testing.T) {
 
 func TestGameOverPayloadRoundTrips(t *testing.T) {
 	uid, gid := uuid.New(), uuid.New()
-	in := notify.GameOver(uid, gid, "won", "120:95:80")
+	summary := notify.GameSummary{ID: gid.String(), Status: "finished", MoveCount: 18, Seats: []notify.SeatStanding{{Seat: 0, Score: 120, IsWinner: true}}}
+	in := notify.GameOver(uid, gid, "won", "120:95:80", summary)
 	if in.UserID != uid || in.Kind != notify.KindGameOver || in.EventID == "" {
 		t.Fatalf("intent metadata wrong: %+v", in)
 	}
@@ -89,19 +94,106 @@ func TestGameOverPayloadRoundTrips(t *testing.T) {
 	if string(ev.GameId()) != gid.String() || string(ev.Result()) != "won" || string(ev.ScoreLine()) != "120:95:80" {
 		t.Fatalf("game_over fields wrong: game=%q result=%q score=%q", ev.GameId(), ev.Result(), ev.ScoreLine())
 	}
+	g := ev.Game(nil)
+	if g == nil || string(g.Id()) != gid.String() || g.MoveCount() != 18 || g.SeatsLength() != 1 {
+		t.Fatalf("final game summary wrong: %+v", g)
+	}
 }
 
 func TestOpponentMovedPayloadRoundTrips(t *testing.T) {
 	uid, gid := uuid.New(), uuid.New()
-	in := notify.OpponentMoved(uid, gid, 1, "play", 24, 130)
+	move := engine.MoveRecord{Player: 1, Action: engine.ActionPlay, Words: []string{"STOOL"}, Score: 24, Total: 130}
+	summary := notify.GameSummary{ID: gid.String(), MoveCount: 9, ToMove: 0, Seats: []notify.SeatStanding{{Seat: 1, Score: 130}}}
+	in := notify.OpponentMoved(uid, gid, move, summary, 42)
 	if in.Kind != notify.KindOpponentMoved {
 		t.Fatalf("kind = %q", in.Kind)
 	}
 	ev := fb.GetRootAsOpponentMovedEvent(in.Payload, 0)
+	// The pre-R4 summary scalars repeat the move.
 	if string(ev.GameId()) != gid.String() || ev.Seat() != 1 || string(ev.Action()) != "play" || ev.Score() != 24 || ev.Total() != 130 {
-		t.Fatalf("decoded wrong: game=%q seat=%d action=%q score=%d total=%d",
+		t.Fatalf("scalars wrong: game=%q seat=%d action=%q score=%d total=%d",
 			ev.GameId(), ev.Seat(), ev.Action(), ev.Score(), ev.Total())
 	}
+	// The R4 delta: the move, the post-move summary and the bag size.
+	if ev.BagLen() != 42 {
+		t.Fatalf("bag_len = %d, want 42", ev.BagLen())
+	}
+	m := ev.Move(nil)
+	if m == nil || m.Player() != 1 || string(m.Action()) != "play" || m.Total() != 130 {
+		t.Fatalf("move wrong: %+v", m)
+	}
+	if g := ev.Game(nil); g == nil || g.MoveCount() != 9 || g.ToMove() != 0 {
+		t.Fatalf("game summary wrong: %+v", ev.Game(nil))
+	}
+}
+
+func TestMatchFoundCarriesInitialState(t *testing.T) {
+	uid, gid := uuid.New(), uuid.New()
+	state := notify.PlayerState{
+		Game:   notify.GameSummary{ID: gid.String(), Variant: "scrabble_en", Seats: []notify.SeatStanding{{Seat: 0, DisplayName: "Ann"}}},
+		Seat:   0,
+		Rack:   []int{0, 1, 2, 255},
+		BagLen: 86,
+	}
+	in := notify.MatchFound(uid, gid, state)
+	if in.UserID != uid || in.Kind != notify.KindMatchFound {
+		t.Fatalf("intent metadata wrong: %+v", in)
+	}
+	ev := fb.GetRootAsMatchFoundEvent(in.Payload, 0)
+	if string(ev.GameId()) != gid.String() {
+		t.Fatalf("game id = %q", ev.GameId())
+	}
+	st := ev.State(nil)
+	if st == nil || st.Seat() != 0 || st.BagLen() != 86 || st.RackLength() != 4 || st.Rack(3) != 255 {
+		t.Fatalf("initial state wrong: %+v", st)
+	}
+	if g := st.Game(nil); g == nil || string(g.Variant()) != "scrabble_en" {
+		t.Fatalf("state game wrong: %+v", st.Game(nil))
+	}
+}
+
+func TestNotificationInvitationCarriesInvitation(t *testing.T) {
+	uid := uuid.New()
+	inv := notify.InvitationSummary{
+		ID:              "inv-1",
+		Inviter:         notify.AccountRef{AccountID: "a-1", DisplayName: "Ann"},
+		Invitees:        []notify.InvitationInvitee{{AccountID: "b-1", DisplayName: "Bob", Seat: 1, Response: "pending"}},
+		Variant:         "erudit_ru",
+		TurnTimeoutSecs: 86400,
+		Status:          "pending",
+	}
+	in := notify.NotificationInvitation(uid, inv)
+	if in.Kind != notify.KindNotification {
+		t.Fatalf("kind = %q", in.Kind)
+	}
+	ev := fb.GetRootAsNotificationEvent(in.Payload, 0)
+	if string(ev.Kind()) != notify.NotifyInvitation {
+		t.Fatalf("sub-kind = %q, want %q", ev.Kind(), notify.NotifyInvitation)
+	}
+	got := ev.Invitation(nil)
+	if got == nil || string(got.Id()) != "inv-1" || string(got.Variant()) != "erudit_ru" || got.InviteesLength() != 1 {
+		t.Fatalf("invitation wrong: %+v", got)
+	}
+	var iv fb.InvitationInvitee
+	if !got.Invitees(&iv, 0) || string(iv.DisplayName()) != "Bob" || iv.Seat() != 1 {
+		t.Fatalf("invitee wrong")
+	}
+	if inviter := got.Inviter(nil); inviter == nil || string(inviter.DisplayName()) != "Ann" {
+		t.Fatalf("inviter wrong")
+	}
+}
+
+func TestNotificationAccountCarriesAccount(t *testing.T) {
+	uid := uuid.New()
+	in := notify.NotificationAccount(uid, notify.NotifyFriendRequest, notify.AccountRef{AccountID: "a-1", DisplayName: "Ann"})
+	ev := fb.GetRootAsNotificationEvent(in.Payload, 0)
+	if string(ev.Kind()) != notify.NotifyFriendRequest {
+		t.Fatalf("sub-kind = %q", ev.Kind())
+	}
+	acc := ev.Account(nil)
+	if acc == nil || string(acc.AccountId()) != "a-1" || string(acc.DisplayName()) != "Ann" {
+		t.Fatalf("account wrong: %+v", acc)
+	}
 }
 
 func TestChatMessagePayloadRoundTrips(t *testing.T) {