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.

gateway/internal/transcode/transcode_test.go

@@ -59,7 +59,7 @@ func TestGameStateRoundTripForwardsUserID(t *testing.T) {
 		if r.URL.Path != "/api/v1/user/games/g-1/state" {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
-		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":1,"seats":[{"seat":0,"account_id":"u-7","score":5}]},"seat":0,"rack":["A","B"],"bag_len":80,"hints_remaining":1}`))
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":1,"seats":[{"seat":0,"account_id":"u-7","score":5}]},"seat":0,"rack":[0,1],"bag_len":80,"hints_remaining":1}`))
 	})
 	defer cleanup()