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/exchange-request.ts

@@ -27,11 +27,9 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-tiles(index: number):string
-tiles(index: number,optionalEncoding:flatbuffers.Encoding):string|Uint8Array
-tiles(index: number,optionalEncoding?:any):string|Uint8Array|null {
+tiles(index: number):number|null {
   const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb!.__vector(this.bb_pos + offset) + index * 4, optionalEncoding) : null;
+  return offset ? this.bb!.readUint8(this.bb!.__vector(this.bb_pos + offset) + index) : 0;
 }
 
 tilesLength():number {
@@ -39,6 +37,11 @@ tilesLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+tilesArray():Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? new Uint8Array(this.bb!.bytes().buffer, this.bb!.bytes().byteOffset + this.bb!.__vector(this.bb_pos + offset), this.bb!.__vector_len(this.bb_pos + offset)) : null;
+}
+
 static startExchangeRequest(builder:flatbuffers.Builder) {
   builder.startObject(2);
 }
@@ -51,16 +54,16 @@ static addTiles(builder:flatbuffers.Builder, tilesOffset:flatbuffers.Offset) {
   builder.addFieldOffset(1, tilesOffset, 0);
 }
 
-static createTilesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
-  builder.startVector(4, data.length, 4);
+static createTilesVector(builder:flatbuffers.Builder, data:number[]|Uint8Array):flatbuffers.Offset {
+  builder.startVector(1, data.length, 1);
   for (let i = data.length - 1; i >= 0; i--) {
-    builder.addOffset(data[i]!);
+    builder.addInt8(data[i]!);
   }
   return builder.endVector();
 }
 
 static startTilesVector(builder:flatbuffers.Builder, numElems:number) {
-  builder.startVector(4, numElems, 4);
+  builder.startVector(1, numElems, 1);
 }
 
 static endExchangeRequest(builder:flatbuffers.Builder):flatbuffers.Offset {