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/ruleset.ts

@@ -22,6 +22,9 @@ export interface VariantRuleset {
   values: readonly number[];
   /** Bag tile count per letter index (how many of each letter the bag holds). */
   counts: readonly number[];
+  /** Display glyph per letter index, upper-cased (matching lib/alphabet.ts). Offline needs these
+   *  to translate the engine's index space to/from the glyphs the game UI shows, with no server. */
+  letters: readonly string[];
 }
 
 /** RULESETS is the static ruleset table, one entry per variant, mirrored from rules.go. */
@@ -34,6 +37,7 @@ export const RULESETS: Record<Variant, VariantRuleset> = {
     //       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
     values: [1, 3, 3, 2, 1, 4, 2, 4, 1, 8, 5, 1, 3, 1, 1, 3, 10, 1, 1, 1, 1, 4, 4, 8, 4, 10],
     counts: [9, 2, 2, 4, 12, 2, 3, 2, 9, 1, 1, 4, 2, 6, 8, 2, 1, 6, 4, 6, 4, 2, 2, 1, 2, 1],
+    letters: [...'ABCDEFGHIJKLMNOPQRSTUVWXYZ'],
   },
   scrabble_ru: {
     size: 33,
@@ -43,6 +47,7 @@ export const RULESETS: Record<Variant, VariantRuleset> = {
     //       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
     values: [1, 3, 1, 3, 2, 1, 3, 5, 5, 1, 4, 2, 2, 2, 1, 1, 2, 1, 1, 1, 2, 10, 5, 5, 5, 8, 10, 10, 4, 3, 8, 8, 3],
     counts: [8, 2, 4, 2, 4, 8, 1, 1, 2, 5, 1, 4, 4, 3, 5, 10, 4, 5, 5, 5, 4, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 2],
+    letters: [...'АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ'],
   },
   erudit_ru: {
     size: 33,
@@ -52,5 +57,6 @@ export const RULESETS: Record<Variant, VariantRuleset> = {
     //       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
     values: [1, 3, 2, 3, 2, 1, 0, 5, 5, 1, 2, 2, 2, 2, 1, 1, 2, 2, 2, 2, 3, 10, 5, 10, 5, 10, 10, 10, 5, 5, 10, 10, 3],
     counts: [10, 3, 5, 3, 5, 9, 0, 2, 2, 8, 4, 6, 4, 5, 8, 10, 6, 6, 6, 5, 3, 1, 2, 1, 2, 1, 1, 1, 2, 2, 1, 1, 3],
+    letters: [...'АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ'],
   },
 };