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/game/Board.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import type { BoardCell } from '../lib/board';
   import type { Premium } from '../lib/premiums';
-  import { tileValue } from '../lib/premiums';
+  import { valueForLetter } from '../lib/alphabet';
   import type { Variant } from '../lib/model';
   import { bonusLabel, type BoardLabelMode } from '../lib/boardlabels';
   import type { Locale } from '../lib/i18n/catalog';
@@ -98,7 +98,7 @@
           >
             {#if letter}
               <span class="letter">{letter}</span>
-              {#if !blank}<span class="val">{tileValue(variant, letter)}</span>{/if}
+              {#if !blank}<span class="val">{valueForLetter(variant, letter)}</span>{/if}
             {:else if r === centre.row && c === centre.col}
               <span class="star">★</span>
             {:else if bl?.kind === 'single'}