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/gen/fbs/scrabblefb/submit-play-request.ts

@@ -2,7 +2,7 @@
 
 import * as flatbuffers from 'flatbuffers';
 
-import { TileRecord } from '../scrabblefb/tile-record.js';
+import { PlayTile } from '../scrabblefb/play-tile.js';
 
 
 export class SubmitPlayRequest {
@@ -37,9 +37,9 @@ dir(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-tiles(index: number, obj?:TileRecord):TileRecord|null {
+tiles(index: number, obj?:PlayTile):PlayTile|null {
   const offset = this.bb!.__offset(this.bb_pos, 8);
-  return offset ? (obj || new TileRecord()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
+  return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {