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/lobby/matchmaker_test.go

@@ -11,6 +11,7 @@ import (
 
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/notify"
 )
 
 // fakeCreator records the games a matchmaker asks it to start.
@@ -27,6 +28,12 @@ func (f *fakeCreator) Create(_ context.Context, p game.CreateParams) (game.Game,
 	return game.Game{ID: uuid.New(), Players: len(p.Seats)}, nil
 }
 
+// InitialState satisfies GameCreator; the matchmaker reads it to enrich match_found. The pairing
+// tests assert on matching behaviour, not the payload, so an empty state is enough.
+func (f *fakeCreator) InitialState(_ context.Context, _, _ uuid.UUID) (notify.PlayerState, error) {
+	return notify.PlayerState{}, nil
+}
+
 // fakeRobots is a RobotProvider returning a fixed robot id, or an error to model
 // an empty pool. It records the variant of the last substitution request.
 type fakeRobots struct {