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/src/lib/client.ts

@@ -67,15 +67,18 @@ export interface GatewayClient {
   lobbyPoll(): Promise<MatchResult>;
 
   // --- game ---
-  gameState(gameId: string): Promise<StateView>;
+  // Stage 13: the play loop exchanges alphabet indices, so submit/evaluate/exchange/
+  // check-word take the game's variant (to map letters<->indices via the cached alphabet
+  // table), and gameState's includeAlphabet asks the server to embed that table.
+  gameState(gameId: string, includeAlphabet: boolean): Promise<StateView>;
   gameHistory(gameId: string): Promise<History>;
-  submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[]): Promise<MoveResult>;
+  submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<MoveResult>;
   pass(gameId: string): Promise<MoveResult>;
-  exchange(gameId: string, tiles: string[]): Promise<MoveResult>;
+  exchange(gameId: string, tiles: string[], variant: Variant): Promise<MoveResult>;
   resign(gameId: string): Promise<MoveResult>;
   hint(gameId: string): Promise<HintResult>;
-  evaluate(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[]): Promise<EvalResult>;
-  checkWord(gameId: string, word: string): Promise<WordCheckResult>;
+  evaluate(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<EvalResult>;
+  checkWord(gameId: string, word: string, variant: Variant): Promise<WordCheckResult>;
   complaint(gameId: string, word: string, note: string): Promise<void>;
 
   // --- chat ---