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.