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-view.ts

@@ -2,6 +2,7 @@
 
 import * as flatbuffers from 'flatbuffers';
 
+import { AlphabetEntry } from '../scrabblefb/alphabet-entry.js';
 import { GameView } from '../scrabblefb/game-view.js';
 
 
@@ -33,11 +34,9 @@ seat():number {
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
 }
 
-rack(index: number):string
-rack(index: number,optionalEncoding:flatbuffers.Encoding):string|Uint8Array
-rack(index: number,optionalEncoding?:any):string|Uint8Array|null {
+rack(index: number):number|null {
   const offset = this.bb!.__offset(this.bb_pos, 8);
-  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;
 }
 
 rackLength():number {
@@ -45,6 +44,11 @@ rackLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+rackArray():Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  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;
+}
+
 bagLen():number {
   const offset = this.bb!.__offset(this.bb_pos, 10);
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
@@ -55,8 +59,18 @@ hintsRemaining():number {
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
 }
 
+alphabet(index: number, obj?:AlphabetEntry):AlphabetEntry|null {
+  const offset = this.bb!.__offset(this.bb_pos, 14);
+  return offset ? (obj || new AlphabetEntry()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
+}
+
+alphabetLength():number {
+  const offset = this.bb!.__offset(this.bb_pos, 14);
+  return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
+}
+
 static startStateView(builder:flatbuffers.Builder) {
-  builder.startObject(5);
+  builder.startObject(6);
 }
 
 static addGame(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset) {
@@ -71,16 +85,16 @@ static addRack(builder:flatbuffers.Builder, rackOffset:flatbuffers.Offset) {
   builder.addFieldOffset(2, rackOffset, 0);
 }
 
-static createRackVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
-  builder.startVector(4, data.length, 4);
+static createRackVector(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 startRackVector(builder:flatbuffers.Builder, numElems:number) {
-  builder.startVector(4, numElems, 4);
+  builder.startVector(1, numElems, 1);
 }
 
 static addBagLen(builder:flatbuffers.Builder, bagLen:number) {
@@ -91,18 +105,35 @@ static addHintsRemaining(builder:flatbuffers.Builder, hintsRemaining:number) {
   builder.addFieldInt32(4, hintsRemaining, 0);
 }
 
+static addAlphabet(builder:flatbuffers.Builder, alphabetOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(5, alphabetOffset, 0);
+}
+
+static createAlphabetVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
+  builder.startVector(4, data.length, 4);
+  for (let i = data.length - 1; i >= 0; i--) {
+    builder.addOffset(data[i]!);
+  }
+  return builder.endVector();
+}
+
+static startAlphabetVector(builder:flatbuffers.Builder, numElems:number) {
+  builder.startVector(4, numElems, 4);
+}
+
 static endStateView(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset, seat:number, rackOffset:flatbuffers.Offset, bagLen:number, hintsRemaining:number):flatbuffers.Offset {
+static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset, seat:number, rackOffset:flatbuffers.Offset, bagLen:number, hintsRemaining:number, alphabetOffset:flatbuffers.Offset):flatbuffers.Offset {
   StateView.startStateView(builder);
   StateView.addGame(builder, gameOffset);
   StateView.addSeat(builder, seat);
   StateView.addRack(builder, rackOffset);
   StateView.addBagLen(builder, bagLen);
   StateView.addHintsRemaining(builder, hintsRemaining);
+  StateView.addAlphabet(builder, alphabetOffset);
   return StateView.endStateView(builder);
 }
 }