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/gen/fbs/scrabblefb.ts

@@ -2,6 +2,7 @@
 
 export { AccountRef } from './scrabblefb/account-ref.js';
 export { Ack } from './scrabblefb/ack.js';
+export { AlphabetEntry } from './scrabblefb/alphabet-entry.js';
 export { BlockList } from './scrabblefb/block-list.js';
 export { ChatList } from './scrabblefb/chat-list.js';
 export { ChatMessage } from './scrabblefb/chat-message.js';
@@ -41,6 +42,7 @@ export { MoveResult } from './scrabblefb/move-result.js';
 export { NotificationEvent } from './scrabblefb/notification-event.js';
 export { NudgeEvent } from './scrabblefb/nudge-event.js';
 export { OpponentMovedEvent } from './scrabblefb/opponent-moved-event.js';
+export { PlayTile } from './scrabblefb/play-tile.js';
 export { Profile } from './scrabblefb/profile.js';
 export { RedeemCodeRequest } from './scrabblefb/redeem-code-request.js';
 export { RedeemResult } from './scrabblefb/redeem-result.js';