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/state-request.ts

@@ -27,22 +27,32 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
+includeAlphabet():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
 static startStateRequest(builder:flatbuffers.Builder) {
-  builder.startObject(1);
+  builder.startObject(2);
 }
 
 static addGameId(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, gameIdOffset, 0);
 }
 
+static addIncludeAlphabet(builder:flatbuffers.Builder, includeAlphabet:boolean) {
+  builder.addFieldInt8(1, +includeAlphabet, +false);
+}
+
 static endStateRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createStateRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createStateRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, includeAlphabet:boolean):flatbuffers.Offset {
   StateRequest.startStateRequest(builder);
   StateRequest.addGameId(builder, gameIdOffset);
+  StateRequest.addIncludeAlphabet(builder, includeAlphabet);
   return StateRequest.endStateRequest(builder);
 }
 }