Stage 13: alphabet on the wire (UI alphabet-agnostic, TODO-4)

Live play now exchanges per-variant alphabet indices instead of concrete
letters (rack out; submit-play, evaluate, exchange, word-check in). The client
caches each variant's (index, letter, value) table behind
StateRequest.include_alphabet and renders the rack and blank chooser from it,
dropping the hardcoded value/alphabet tables. History, the durable journal and
GCG stay decoded concrete characters (ARCHITECTURE §9.1, unchanged).

- pkg/fbs: new AlphabetEntry + PlayTile; StateView.rack -> [ubyte] + alphabet;
  StateRequest.include_alphabet; SubmitPlay/Eval tiles -> [PlayTile];
  Exchange tiles + CheckWord word -> [ubyte] (committed Go + TS regenerated).
- engine: AlphabetTable + a cached per-variant codec (LetterForIndex/EncodeRack/
  DecodeTiles/DecodeWord) + BlankIndex sentinel; Go parity test.
- backend server edge maps index<->letter (new thin game.Service.GameVariant);
  game.Service domain methods, engine.Game and the robot keep one letter-based
  play path. The gateway forwards indices verbatim (no alphabet table).
- ui: lib/alphabet.ts in-memory cache; codec encodes/decodes indices; premiums.ts
  is geometry-only; the mock seeds a fixture table; the UI normalises display to
  upper case (codec + cache), leaving placement/board/checkword unchanged.

Parity moved to the Go engine.AlphabetTable test; premiums.ts loses its value
tables. Discharges TODO-4.

ui/README.md

@@ -42,9 +42,14 @@ out of production). Both speak the plain `lib/model.ts` types via `lib/codec.ts`
 
 **No board on the wire:** `StateView` is a summary + rack only, so the client
 reconstructs the 15×15 board by replaying the decoded move journal (`game.history`).
-Premium squares and tile values (`lib/premiums.ts`) are a client-side map **ported from
-`scrabble-solver/rules/rules.go`** (pinned by a Vitest parity test). Board, tiles and
-effects are pure CSS + Unicode — no image/font/SVG assets.
+**The play loop is alphabet-agnostic (Stage 13):** the rack and the play / exchange /
+word-check requests carry **alphabet indices**, and the client caches each variant's
+`(index, letter, value)` table — sent once behind `StateRequest.include_alphabet` — in
+`lib/alphabet.ts`, rendering the rack and blank chooser from it. **Premium squares**
+(`lib/premiums.ts`) stay a client-side geometry map **ported from
+`scrabble-solver/rules/rules.go`** (pinned by a Vitest parity test); **tile values and the
+alphabet now come from the server table** (their parity lives in the Go `engine.AlphabetTable`
+test). Board, tiles and effects are pure CSS + Unicode — no image/font/SVG assets.
 
 ## Codegen
 
@@ -65,7 +70,8 @@ runtime; the Telegram SDK itself is wired in the Telegram stage.
 ```
 src/
   lib/         model, client facade, transport (+ mock), codec, board replay,
-               placement state machine, premiums, stats, share, i18n, theme, session, router, app store
+               placement state machine, premiums (geometry), alphabet cache, stats, share,
+               i18n, theme, session, router, app store
   components/  Header, Menu (+ badge), Modal, Toast, TabBar, Screen
   screens/     Login, Lobby, NewGame, Profile, Settings, About, Friends, Stats
   game/        Game, Board, Rack, Controls, MakeMove, Chat