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/transport.ts

@@ -81,20 +81,20 @@ export function createTransport(baseUrl: string): GatewayClient {
       return codec.decodeMatchResult(await exec('lobby.poll', codec.empty()));
     },
 
-    async gameState(id) {
-      return codec.decodeStateView(await exec('game.state', codec.encodeStateRequest(id)));
+    async gameState(id, includeAlphabet) {
+      return codec.decodeStateView(await exec('game.state', codec.encodeStateRequest(id, includeAlphabet)));
     },
     async gameHistory(id) {
       return codec.decodeHistory(await exec('game.history', codec.encodeGameAction(id)));
     },
-    async submitPlay(id, dir, tiles) {
-      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, dir, tiles)));
+    async submitPlay(id, dir, tiles, variant) {
+      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, dir, tiles, variant)));
     },
     async pass(id) {
       return codec.decodeMoveResult(await exec('game.pass', codec.encodeGameAction(id)));
     },
-    async exchange(id, tiles) {
-      return codec.decodeMoveResult(await exec('game.exchange', codec.encodeExchange(id, tiles)));
+    async exchange(id, tiles, variant) {
+      return codec.decodeMoveResult(await exec('game.exchange', codec.encodeExchange(id, tiles, variant)));
     },
     async resign(id) {
       return codec.decodeMoveResult(await exec('game.resign', codec.encodeGameAction(id)));
@@ -102,11 +102,11 @@ export function createTransport(baseUrl: string): GatewayClient {
     async hint(id) {
       return codec.decodeHintResult(await exec('game.hint', codec.encodeGameAction(id)));
     },
-    async evaluate(id, dir, tiles) {
-      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, dir, tiles)));
+    async evaluate(id, dir, tiles, variant) {
+      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, dir, tiles, variant)));
     },
-    async checkWord(id, word) {
-      return codec.decodeWordCheck(await exec('game.check_word', codec.encodeCheckWord(id, word)));
+    async checkWord(id, word, variant) {
+      return codec.decodeWordCheck(await exec('game.check_word', codec.encodeCheckWord(id, word, variant)));
     },
     async complaint(id, word, note) {
       await exec('game.complaint', codec.encodeComplaint(id, word, note));