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/StateView.go

@@ -66,13 +66,13 @@ func (rcv *StateView) MutateSeat(n int32) bool {
 	return rcv._tab.MutateInt32Slot(6, n)
 }
 
-func (rcv *StateView) Rack(j int) []byte {
+func (rcv *StateView) Rack(j int) byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
 	if o != 0 {
 		a := rcv._tab.Vector(o)
-		return rcv._tab.ByteVector(a + flatbuffers.UOffsetT(j*4))
+		return rcv._tab.GetByte(a + flatbuffers.UOffsetT(j*1))
 	}
-	return nil
+	return 0
 }
 
 func (rcv *StateView) RackLength() int {
@@ -83,6 +83,23 @@ func (rcv *StateView) RackLength() int {
 	return 0
 }
 
+func (rcv *StateView) RackBytes() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *StateView) MutateRack(j int, n byte) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.MutateByte(a+flatbuffers.UOffsetT(j*1), n)
+	}
+	return false
+}
+
 func (rcv *StateView) BagLen() int32 {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
 	if o != 0 {
@@ -107,8 +124,28 @@ func (rcv *StateView) MutateHintsRemaining(n int32) bool {
 	return rcv._tab.MutateInt32Slot(12, n)
 }
 
+func (rcv *StateView) Alphabet(obj *AlphabetEntry, j int) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		x := rcv._tab.Vector(o)
+		x += flatbuffers.UOffsetT(j) * 4
+		x = rcv._tab.Indirect(x)
+		obj.Init(rcv._tab.Bytes, x)
+		return true
+	}
+	return false
+}
+
+func (rcv *StateView) AlphabetLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
 func StateViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(5)
+	builder.StartObject(6)
 }
 func StateViewAddGame(builder *flatbuffers.Builder, game flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(game), 0)
@@ -120,7 +157,7 @@ func StateViewAddRack(builder *flatbuffers.Builder, rack flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(2, flatbuffers.UOffsetT(rack), 0)
 }
 func StateViewStartRackVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
-	return builder.StartVector(4, numElems, 4)
+	return builder.StartVector(1, numElems, 1)
 }
 func StateViewAddBagLen(builder *flatbuffers.Builder, bagLen int32) {
 	builder.PrependInt32Slot(3, bagLen, 0)
@@ -128,6 +165,12 @@ func StateViewAddBagLen(builder *flatbuffers.Builder, bagLen int32) {
 func StateViewAddHintsRemaining(builder *flatbuffers.Builder, hintsRemaining int32) {
 	builder.PrependInt32Slot(4, hintsRemaining, 0)
 }
+func StateViewAddAlphabet(builder *flatbuffers.Builder, alphabet flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(5, flatbuffers.UOffsetT(alphabet), 0)
+}
+func StateViewStartAlphabetVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
 func StateViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }