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/play-tile.ts

@@ -0,0 +1,76 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+export class PlayTile {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):PlayTile {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsPlayTile(bb:flatbuffers.ByteBuffer, obj?:PlayTile):PlayTile {
+  return (obj || new PlayTile()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsPlayTile(bb:flatbuffers.ByteBuffer, obj?:PlayTile):PlayTile {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new PlayTile()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+row():number {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+col():number {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+letter():number {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  return offset ? this.bb!.readUint8(this.bb_pos + offset) : 0;
+}
+
+blank():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 10);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
+static startPlayTile(builder:flatbuffers.Builder) {
+  builder.startObject(4);
+}
+
+static addRow(builder:flatbuffers.Builder, row:number) {
+  builder.addFieldInt32(0, row, 0);
+}
+
+static addCol(builder:flatbuffers.Builder, col:number) {
+  builder.addFieldInt32(1, col, 0);
+}
+
+static addLetter(builder:flatbuffers.Builder, letter:number) {
+  builder.addFieldInt8(2, letter, 0);
+}
+
+static addBlank(builder:flatbuffers.Builder, blank:boolean) {
+  builder.addFieldInt8(3, +blank, +false);
+}
+
+static endPlayTile(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createPlayTile(builder:flatbuffers.Builder, row:number, col:number, letter:number, blank:boolean):flatbuffers.Offset {
+  PlayTile.startPlayTile(builder);
+  PlayTile.addRow(builder, row);
+  PlayTile.addCol(builder, col);
+  PlayTile.addLetter(builder, letter);
+  PlayTile.addBlank(builder, blank);
+  return PlayTile.endPlayTile(builder);
+}
+}