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).

ui/src/lib/localgame/testdata/rulesets.json

@@ -73,6 +73,41 @@
       1,
       1,
       3
+    ],
+    "letters": [
+      "А",
+      "Б",
+      "В",
+      "Г",
+      "Д",
+      "Е",
+      "Ё",
+      "Ж",
+      "З",
+      "И",
+      "Й",
+      "К",
+      "Л",
+      "М",
+      "Н",
+      "О",
+      "П",
+      "Р",
+      "С",
+      "Т",
+      "У",
+      "Ф",
+      "Х",
+      "Ц",
+      "Ч",
+      "Ш",
+      "Щ",
+      "Ъ",
+      "Ы",
+      "Ь",
+      "Э",
+      "Ю",
+      "Я"
     ]
   },
   "scrabble_en": {
@@ -135,6 +170,34 @@
       1,
       2,
       1
+    ],
+    "letters": [
+      "A",
+      "B",
+      "C",
+      "D",
+      "E",
+      "F",
+      "G",
+      "H",
+      "I",
+      "J",
+      "K",
+      "L",
+      "M",
+      "N",
+      "O",
+      "P",
+      "Q",
+      "R",
+      "S",
+      "T",
+      "U",
+      "V",
+      "W",
+      "X",
+      "Y",
+      "Z"
     ]
   },
   "scrabble_ru": {
@@ -211,6 +274,41 @@
       1,
       1,
       2
+    ],
+    "letters": [
+      "А",
+      "Б",
+      "В",
+      "Г",
+      "Д",
+      "Е",
+      "Ё",
+      "Ж",
+      "З",
+      "И",
+      "Й",
+      "К",
+      "Л",
+      "М",
+      "Н",
+      "О",
+      "П",
+      "Р",
+      "С",
+      "Т",
+      "У",
+      "Ф",
+      "Х",
+      "Ц",
+      "Ч",
+      "Ш",
+      "Щ",
+      "Ъ",
+      "Ы",
+      "Ь",
+      "Э",
+      "Ю",
+      "Я"
     ]
   }
 }