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.

pkg/fbs/scrabblefb/StateRequest.go

@@ -49,12 +49,27 @@ func (rcv *StateRequest) GameId() []byte {
 	return nil
 }
 
+func (rcv *StateRequest) IncludeAlphabet() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *StateRequest) MutateIncludeAlphabet(n bool) bool {
+	return rcv._tab.MutateBoolSlot(6, n)
+}
+
 func StateRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(1)
+	builder.StartObject(2)
 }
 func StateRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(gameId), 0)
 }
+func StateRequestAddIncludeAlphabet(builder *flatbuffers.Builder, includeAlphabet bool) {
+	builder.PrependBoolSlot(1, includeAlphabet, false)
+}
 func StateRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }