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.

ui/README.md

@@ -40,7 +40,9 @@ The build has **two entries**: the game SPA (`index.html`, served at `/app/` and
 A single Connect `Execute(message_type, payload)` carries every unary op; the request
 and response bodies are **FlatBuffers** tables (`pkg/fbs/scrabble.fbs`) in `payload`.
 The session token rides in `Authorization: Bearer`; a domain failure comes back in
-`result_code`. `Subscribe` is the live event stream. `lib/transport.ts` is the real
+`result_code`. `Subscribe` is the live event stream; R4 made its game events carry a state **delta**
+that `lib/gamedelta.ts` applies to the per-game cache (`lib/gamecache.ts`), so a move renders without
+a follow-up `game.state` (a gap falls back to a refetch). `lib/transport.ts` is the real
 client; `lib/mock/` is an in-memory fake selected by `MODE === 'mock'` (and tree-shaken
 out of production). Both speak the plain `lib/model.ts` types via `lib/codec.ts`.