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/AlphabetEntry.go

@@ -0,0 +1,90 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type AlphabetEntry struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsAlphabetEntry(buf []byte, offset flatbuffers.UOffsetT) *AlphabetEntry {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &AlphabetEntry{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishAlphabetEntryBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsAlphabetEntry(buf []byte, offset flatbuffers.UOffsetT) *AlphabetEntry {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &AlphabetEntry{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedAlphabetEntryBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *AlphabetEntry) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *AlphabetEntry) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *AlphabetEntry) Index() byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.GetByte(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *AlphabetEntry) MutateIndex(n byte) bool {
+	return rcv._tab.MutateByteSlot(4, n)
+}
+
+func (rcv *AlphabetEntry) Letter() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *AlphabetEntry) Value() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *AlphabetEntry) MutateValue(n int32) bool {
+	return rcv._tab.MutateInt32Slot(8, n)
+}
+
+func AlphabetEntryStart(builder *flatbuffers.Builder) {
+	builder.StartObject(3)
+}
+func AlphabetEntryAddIndex(builder *flatbuffers.Builder, index byte) {
+	builder.PrependByteSlot(0, index, 0)
+}
+func AlphabetEntryAddLetter(builder *flatbuffers.Builder, letter flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(letter), 0)
+}
+func AlphabetEntryAddValue(builder *flatbuffers.Builder, value int32) {
+	builder.PrependInt32Slot(2, value, 0)
+}
+func AlphabetEntryEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}