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/scrabble.fbs

@@ -14,8 +14,9 @@ namespace scrabblefb;
 
 // --- shared building blocks ---
 
-// TileRecord is one placed (or to-place) tile: its board coordinate, the concrete
-// letter ("?" when read from a hand for a blank) and whether it came from a blank.
+// TileRecord is one tile in a decoded move record (history, move result, hint): its
+// board coordinate, the concrete letter ("?" when read from a hand for a blank) and
+// whether it came from a blank. Inbound tiles to place use PlayTile (alphabet indices).
 table TileRecord {
   row:int;
   col:int;
@@ -23,6 +24,25 @@ table TileRecord {
   blank:bool;
 }
 
+// PlayTile is one inbound tile to place, addressed by its alphabet index rather than a
+// concrete letter (Stage 13). For a blank, letter carries the designated letter's index
+// and blank is true. The board coordinate is its target square.
+table PlayTile {
+  row:int;
+  col:int;
+  letter:ubyte;
+  blank:bool;
+}
+
+// AlphabetEntry is one letter of a variant's alphabet, sent for display only (Stage 13):
+// index is the engine alphabet-index byte the wire uses for this letter, letter is the
+// concrete character and value is its tile score. The client caches the table per variant.
+table AlphabetEntry {
+  index:ubyte;
+  letter:string;
+  value:int;
+}
+
 // SeatView is one seat's public standing in a game. display_name is resolved by the
 // backend from the account store (added trailing — backward-compatible).
 table SeatView {
@@ -123,11 +143,12 @@ table Profile {
 
 // --- game (authenticated) ---
 
-// SubmitPlayRequest places tiles in a direction on the player's turn.
+// SubmitPlayRequest places tiles in a direction on the player's turn. tiles are addressed
+// by alphabet index (Stage 13).
 table SubmitPlayRequest {
   game_id:string;
   dir:string;
-  tiles:[TileRecord];
+  tiles:[PlayTile];
 }
 
 // MoveResult is the outcome of a committed move: the move and the post-move game.
@@ -136,19 +157,25 @@ table MoveResult {
   game:GameView;
 }
 
-// StateRequest asks for the requesting player's view of a game.
+// StateRequest asks for the requesting player's view of a game. include_alphabet asks the
+// backend to embed the variant's AlphabetEntry table in the reply (Stage 13); the client
+// sets it only on a per-variant cache miss so the table is not resent on every poll.
 table StateRequest {
   game_id:string;
+  include_alphabet:bool = false;
 }
 
-// StateView is a player's view of a game: the shared summary plus their private
-// rack, the bag size and their remaining hint budget.
+// StateView is a player's view of a game: the shared summary plus their private rack, the
+// bag size and their remaining hint budget. rack carries alphabet indices (Stage 13); a
+// blank tile is the sentinel index 255. alphabet is present only when the request set
+// include_alphabet (a display table the client caches per variant).
 table StateView {
   game:GameView;
   seat:int;
-  rack:[string];
+  rack:[ubyte];
   bag_len:int;
   hints_remaining:int;
+  alphabet:[AlphabetEntry];
 }
 
 // GameActionRequest carries just a game id (pass / resign / hint / history).
@@ -156,17 +183,19 @@ table GameActionRequest {
   game_id:string;
 }
 
-// ExchangeRequest swaps the listed rack tiles back into the bag.
+// ExchangeRequest swaps the listed rack tiles back into the bag. tiles are alphabet
+// indices (Stage 13); a blank is the sentinel index 255.
 table ExchangeRequest {
   game_id:string;
-  tiles:[string];
+  tiles:[ubyte];
 }
 
-// EvalRequest previews a tentative play without committing it.
+// EvalRequest previews a tentative play without committing it. tiles are addressed by
+// alphabet index (Stage 13).
 table EvalRequest {
   game_id:string;
   dir:string;
-  tiles:[TileRecord];
+  tiles:[PlayTile];
 }
 
 // EvalResult is an unlimited move preview: legality, score and the words formed.
@@ -176,10 +205,11 @@ table EvalResult {
   words:[string];
 }
 
-// CheckWordRequest looks a word up in the game's pinned dictionary.
+// CheckWordRequest looks a word up in the game's pinned dictionary. word is a sequence of
+// alphabet indices (Stage 13); the client constrains input to the variant's alphabet.
 table CheckWordRequest {
   game_id:string;
-  word:string;
+  word:[ubyte];
 }
 
 // WordCheckResult is the dictionary lookup outcome.

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()
+}

pkg/fbs/scrabblefb/CheckWordRequest.go

@@ -49,7 +49,24 @@ func (rcv *CheckWordRequest) GameId() []byte {
 	return nil
 }
 
-func (rcv *CheckWordRequest) Word() []byte {
+func (rcv *CheckWordRequest) Word(j int) byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.GetByte(a + flatbuffers.UOffsetT(j*1))
+	}
+	return 0
+}
+
+func (rcv *CheckWordRequest) WordLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
+func (rcv *CheckWordRequest) WordBytes() []byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
 	if o != 0 {
 		return rcv._tab.ByteVector(o + rcv._tab.Pos)
@@ -57,6 +74,15 @@ func (rcv *CheckWordRequest) Word() []byte {
 	return nil
 }
 
+func (rcv *CheckWordRequest) MutateWord(j int, n byte) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.MutateByte(a+flatbuffers.UOffsetT(j*1), n)
+	}
+	return false
+}
+
 func CheckWordRequestStart(builder *flatbuffers.Builder) {
 	builder.StartObject(2)
 }
@@ -66,6 +92,9 @@ func CheckWordRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.
 func CheckWordRequestAddWord(builder *flatbuffers.Builder, word flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(word), 0)
 }
+func CheckWordRequestStartWordVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(1, numElems, 1)
+}
 func CheckWordRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }

pkg/fbs/scrabblefb/EvalRequest.go

@@ -57,7 +57,7 @@ func (rcv *EvalRequest) Dir() []byte {
 	return nil
 }
 
-func (rcv *EvalRequest) Tiles(obj *TileRecord, j int) bool {
+func (rcv *EvalRequest) Tiles(obj *PlayTile, j int) bool {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
 	if o != 0 {
 		x := rcv._tab.Vector(o)

pkg/fbs/scrabblefb/ExchangeRequest.go

@@ -49,13 +49,13 @@ func (rcv *ExchangeRequest) GameId() []byte {
 	return nil
 }
 
-func (rcv *ExchangeRequest) Tiles(j int) []byte {
+func (rcv *ExchangeRequest) Tiles(j int) byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
 	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 *ExchangeRequest) TilesLength() int {
@@ -66,6 +66,23 @@ func (rcv *ExchangeRequest) TilesLength() int {
 	return 0
 }
 
+func (rcv *ExchangeRequest) TilesBytes() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *ExchangeRequest) MutateTiles(j int, n byte) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.MutateByte(a+flatbuffers.UOffsetT(j*1), n)
+	}
+	return false
+}
+
 func ExchangeRequestStart(builder *flatbuffers.Builder) {
 	builder.StartObject(2)
 }
@@ -76,7 +93,7 @@ func ExchangeRequestAddTiles(builder *flatbuffers.Builder, tiles flatbuffers.UOf
 	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(tiles), 0)
 }
 func ExchangeRequestStartTilesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
-	return builder.StartVector(4, numElems, 4)
+	return builder.StartVector(1, numElems, 1)
 }
 func ExchangeRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()

pkg/fbs/scrabblefb/PlayTile.go

@@ -0,0 +1,109 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type PlayTile struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsPlayTile(buf []byte, offset flatbuffers.UOffsetT) *PlayTile {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &PlayTile{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishPlayTileBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsPlayTile(buf []byte, offset flatbuffers.UOffsetT) *PlayTile {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &PlayTile{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedPlayTileBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *PlayTile) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *PlayTile) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *PlayTile) Row() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *PlayTile) MutateRow(n int32) bool {
+	return rcv._tab.MutateInt32Slot(4, n)
+}
+
+func (rcv *PlayTile) Col() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *PlayTile) MutateCol(n int32) bool {
+	return rcv._tab.MutateInt32Slot(6, n)
+}
+
+func (rcv *PlayTile) Letter() byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetByte(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *PlayTile) MutateLetter(n byte) bool {
+	return rcv._tab.MutateByteSlot(8, n)
+}
+
+func (rcv *PlayTile) Blank() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *PlayTile) MutateBlank(n bool) bool {
+	return rcv._tab.MutateBoolSlot(10, n)
+}
+
+func PlayTileStart(builder *flatbuffers.Builder) {
+	builder.StartObject(4)
+}
+func PlayTileAddRow(builder *flatbuffers.Builder, row int32) {
+	builder.PrependInt32Slot(0, row, 0)
+}
+func PlayTileAddCol(builder *flatbuffers.Builder, col int32) {
+	builder.PrependInt32Slot(1, col, 0)
+}
+func PlayTileAddLetter(builder *flatbuffers.Builder, letter byte) {
+	builder.PrependByteSlot(2, letter, 0)
+}
+func PlayTileAddBlank(builder *flatbuffers.Builder, blank bool) {
+	builder.PrependBoolSlot(3, blank, false)
+}
+func PlayTileEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}

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()
 }

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()
 }

pkg/fbs/scrabblefb/SubmitPlayRequest.go

@@ -57,7 +57,7 @@ func (rcv *SubmitPlayRequest) Dir() []byte {
 	return nil
 }
 
-func (rcv *SubmitPlayRequest) Tiles(obj *TileRecord, j int) bool {
+func (rcv *SubmitPlayRequest) Tiles(obj *PlayTile, j int) bool {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
 	if o != 0 {
 		x := rcv._tab.Vector(o)