feat(offline): local game source (Phase B3.1)

A GatewayClient-shaped facade over the offline engine, so the same game screen can drive
a local vs_ai game with no backend (the wiring into Game.svelte is B3.2).

- source.ts: LocalSource implements the game-loop subset (GameLoopSource) for a local game
  id — gameState/gameHistory via replay, submitPlay/pass/exchange/resign apply the human
  move then run the robot (decide(generateMoves)) synchronously, persisting both and
  delivering the robot's move through a per-game event emitter (no live stream). hint is
  gated to >30 min since the robot's last move; evaluate/checkWord are local. It translates
  the UI's glyph space to the engine's index space with the static letters table.
- ruleset.ts: add the static per-variant letters (glyphs), pinned to the Go alphabet —
  offline is now fully self-contained (no reliance on a warm server alphabet cache).
- engine.ts: submitPlay (infers the direction like the server SubmitPlay), evaluatePlay +
  dictionaryHas for the move preview / word check, and record the main-word coordinate +
  the words on a play (for the history MoveRecord).
- source.test.ts: create -> human pass -> synchronous robot reply via the event, the hint
  gate, decoded history, and a whole game driven to completion.

Pure additive library code; no runtime behavior change (bundle unchanged).

backend/cmd/movegen/main.go

@@ -25,6 +25,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"gitea.iliadenisov.ru/developer/scrabble-solver/board"
 	"gitea.iliadenisov.ru/developer/scrabble-solver/rack"
@@ -370,12 +371,13 @@ func russianRack(letters string, blanks int) genRack {
 // parity test can pin that hand-copied table to the Go rulesets (scrabble-solver/rules).
 func emitRulesets() {
 	type rsFix struct {
-		Size     int   `json:"size"`
-		RackSize int   `json:"rackSize"`
-		Bingo    int   `json:"bingo"`
-		Blanks   int   `json:"blanks"`
-		Values   []int `json:"values"`
-		Counts   []int `json:"counts"`
+		Size     int      `json:"size"`
+		RackSize int      `json:"rackSize"`
+		Bingo    int      `json:"bingo"`
+		Blanks   int      `json:"blanks"`
+		Values   []int    `json:"values"`
+		Counts   []int    `json:"counts"`
+		Letters  []string `json:"letters"`
 	}
 	out := map[string]rsFix{}
 	for _, v := range []struct {
@@ -386,7 +388,15 @@ func emitRulesets() {
 		{"scrabble_ru", rules.RussianScrabble()},
 		{"erudit_ru", rules.Erudit()},
 	} {
-		out[v.name] = rsFix{Size: v.rs.Size(), RackSize: v.rs.RackSize, Bingo: v.rs.Bingo, Blanks: v.rs.Blanks, Values: v.rs.Values, Counts: v.rs.Counts}
+		letters := make([]string, v.rs.Size())
+		for i := range letters {
+			ch, err := v.rs.Alphabet.Character(byte(i))
+			if err != nil {
+				log.Fatalf("movegen: %s letter %d: %v", v.name, i, err)
+			}
+			letters[i] = strings.ToUpper(ch)
+		}
+		out[v.name] = rsFix{Size: v.rs.Size(), RackSize: v.rs.RackSize, Bingo: v.rs.Bingo, Blanks: v.rs.Blanks, Values: v.rs.Values, Counts: v.rs.Counts, Letters: letters}
 	}
 	dir := filepath.Join("ui", "src", "lib", "localgame", "testdata")
 	if err := os.MkdirAll(dir, 0o755); err != nil {