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/Game.svelte

@@ -14,7 +14,8 @@
   import { t } from '../lib/i18n/index.svelte';
   import type { ChatMessage, Direction, EvalResult, MoveRecord, StateView } from '../lib/model';
   import { replay } from '../lib/board';
-  import { alphabet, centre, premiumGrid } from '../lib/premiums';
+  import { centre, premiumGrid } from '../lib/premiums';
+  import { alphabetLetters, hasAlphabet } from '../lib/alphabet';
   import { canCheckWord, sanitizeCheckWord } from '../lib/checkword';
   import { shareOrDownloadGcg } from '../lib/share';
   import {
@@ -84,7 +85,13 @@
 
   async function load() {
     try {
-      const [st, hist] = await Promise.all([gateway.gameState(id), gateway.gameHistory(id)]);
+      // Ask for the alphabet table only on a per-variant cache miss (the first open of a
+      // game whose variant the client has not cached yet); steady-state polls omit it.
+      const includeAlphabet = !view || !hasAlphabet(view.game.variant);
+      const [st, hist] = await Promise.all([
+        gateway.gameState(id, includeAlphabet),
+        gateway.gameHistory(id),
+      ]);
       view = st;
       moves = hist.moves;
       placement = newPlacement(st.rack);
@@ -206,7 +213,7 @@
     if (!sub) return;
     previewTimer = setTimeout(async () => {
       try {
-        preview = await gateway.evaluate(id, sub.dir, sub.tiles);
+        preview = await gateway.evaluate(id, sub.dir, sub.tiles, variant);
       } catch {
         /* best-effort */
       }
@@ -218,7 +225,7 @@
     if (!sub) return;
     busy = true;
     try {
-      await gateway.submitPlay(id, sub.dir, sub.tiles);
+      await gateway.submitPlay(id, sub.dir, sub.tiles, variant);
       zoomed = false;
       await load();
     } catch (e) {
@@ -298,7 +305,7 @@
     exchangeOpen = false;
     busy = true;
     try {
-      await gateway.exchange(id, tiles);
+      await gateway.exchange(id, tiles, variant);
       await load();
     } catch (e) {
       handleError(e);
@@ -313,7 +320,7 @@
     checkOpen = true;
   }
   function onCheckInput(e: Event) {
-    checkWord = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabet(variant));
+    checkWord = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabetLetters(variant));
   }
   // Check is disabled while cooling down, for an already-checked word, or an out-of-range
   // length. The input filter already restricts to the variant's alphabet.
@@ -326,9 +333,11 @@
     cooling = true;
     setTimeout(() => (cooling = false), 5000);
     try {
-      const r = await gateway.checkWord(id, w);
-      checkedWords.set(r.word.toUpperCase(), r.legal);
-      checkResult = r;
+      const r = await gateway.checkWord(id, w, variant);
+      // Key the cache and the displayed result on the upper-case word the player typed; the
+      // server echoes the decoded concrete word in the solver's lower case.
+      checkedWords.set(w, r.legal);
+      checkResult = { word: w, legal: r.legal };
     } catch (e) {
       handleError(e);
     }
@@ -535,7 +544,7 @@
 {#if blankPrompt}
   <Modal title={t('game.chooseBlank')} onclose={() => (blankPrompt = null)}>
     <div class="alpha">
-      {#each alphabet(variant) as ch (ch)}
+      {#each alphabetLetters(variant) as ch (ch)}
         <button onclick={() => chooseBlank(ch)}>{ch}</button>
       {/each}
     </div>