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/premiums.test.ts

@@ -1,8 +1,10 @@
 import { describe, expect, it } from 'vitest';
-import { alphabet, BOARD_SIZE, centre, premiumGrid, tileValue } from './premiums';
+import { BOARD_SIZE, centre, premiumGrid } from './premiums';
 
-// Parity with scrabble-solver/rules/rules.go: english/russian share standardBoard
-// (centre is a double word); erudit shares the geometry but a non-doubling centre.
+// Premium-square geometry parity with scrabble-solver/rules/rules.go: english/russian
+// share standardBoard (centre is a double word); erudit shares the geometry but a
+// non-doubling centre. Tile-value and alphabet parity moved to the Go engine test
+// (backend/internal/engine AlphabetTable) in Stage 13 — the server now owns that table.
 describe('premium layout', () => {
   it('is a 15x15 grid with TW corners', () => {
     const g = premiumGrid('english');
@@ -35,19 +37,3 @@ describe('premium layout', () => {
     expect(count('DW')).toBe(17); // 16 double-word squares + the centre
   });
 });
-
-describe('tile values', () => {
-  it('scores letters per variant and zero for a blank', () => {
-    expect(tileValue('english', 'A')).toBe(1);
-    expect(tileValue('english', 'Q')).toBe(10);
-    expect(tileValue('english', '?')).toBe(0);
-    expect(tileValue('russian', 'Ф')).toBe(10);
-    expect(tileValue('erudit', 'Ё')).toBe(0);
-  });
-
-  it('exposes the full alphabet for the blank chooser', () => {
-    expect(alphabet('english')).toHaveLength(26);
-    expect(alphabet('russian')).toHaveLength(33);
-    expect(alphabet('erudit')).toHaveLength(33);
-  });
-});