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.ts

@@ -1,8 +1,9 @@
-// Board premium layout and tile values — ported verbatim from the engine source of
-// truth, scrabble-solver/rules/rules.go (standardBoard / eruditBoard, and the
-// per-variant value tables). These are NOT transmitted on the wire (StateView has
-// no board), so the client renders them locally. A Vitest parity test pins the
-// layout against the known geometry. Keep this in lockstep with the solver.
+// Board premium layout — the 15x15 premium-square geometry, ported from the engine source
+// of truth, scrabble-solver/rules/rules.go (standardBoard / eruditBoard). The board is not
+// transmitted on the wire (StateView has no board), so the client renders the premiums
+// locally; only the centre differs by variant. A Vitest parity test pins the geometry.
+// Tile values and the alphabet moved to the server-sent per-variant table in Stage 13 (see
+// lib/alphabet.ts), so this file is geometry only.
 
 import type { Variant } from './model';
 
@@ -84,43 +85,5 @@ export function centre(variant: Variant): { row: number; col: number } {
   return { row: 7, col: 7 };
 }
 
-// --- tile values (points shown on the tile face); blank scores 0 ---
-
-// English Latin a..z (rules.go English()).
-const enValues =
-  'a1 b3 c3 d2 e1 f4 g2 h4 i1 j8 k5 l1 m3 n1 o1 p3 q10 r1 s1 t1 u1 v4 w4 x8 y4 z10';
-// Russian а..я incl. ё (rules.go RussianScrabble()).
-const ruValues =
-  'а1 б3 в1 г3 д2 е1 ё3 ж5 з5 и1 й4 к2 л2 м2 н1 о1 п2 р1 с1 т1 у2 ф10 х5 ц5 ч5 ш8 щ10 ъ10 ы4 ь3 э8 ю8 я3';
-// Эрудит а..я incl. ё=0 (rules.go Erudit()).
-const eruditValues =
-  'а1 б3 в2 г3 д2 е1 ё0 ж5 з5 и1 й2 к2 л2 м2 н1 о1 п2 р2 с2 т2 у3 ф10 х5 ц10 ч5 ш10 щ10 ъ10 ы5 ь5 э10 ю10 я3';
-
-// Split each "letter+value" token into its letter (all but trailing digits) and its
-// integer value (the trailing digits).
-function valueTable(spec: string): Map<string, number> {
-  const m = new Map<string, number>();
-  for (const pair of spec.trim().split(/\s+/)) {
-    const match = pair.match(/^(.+?)(\d+)$/);
-    if (!match) continue;
-    m.set(match[1].toUpperCase(), Number(match[2]));
-  }
-  return m;
-}
-
-const VALUES: Record<Variant, Map<string, number>> = {
-  english: valueTable(enValues),
-  russian: valueTable(ruValues),
-  erudit: valueTable(eruditValues),
-};
-
-/** tileValue returns the point value of a concrete letter; a blank ("?") is 0. */
-export function tileValue(variant: Variant, letter: string): number {
-  if (!letter || letter === '?') return 0;
-  return VALUES[variant]?.get(letter.toUpperCase()) ?? 0;
-}
-
-/** alphabet lists a variant's letters in dictionary order (used by the blank chooser). */
-export function alphabet(variant: Variant): string[] {
-  return [...VALUES[variant].keys()];
-}
+// Tile values and the per-variant alphabet now arrive from the server (lib/alphabet.ts,
+// Stage 13); the board geometry above is all this module owns.