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/engine.ts

@@ -14,6 +14,7 @@ import { Bag } from './bag';
 import { RULESETS } from './ruleset';
 import { generateMoves, GenRack, Both } from '../dict/generate';
 import { validatePlay, type Direction, type Placement, type Ruleset, type Move } from '../dict/validate';
+import { playDirection } from '../dict/direction';
 import { premiumGrid, centre, BOARD_SIZE, RACK_SIZE, BINGO, type Premium } from '../premiums';
 import { BLANK_INDEX } from '../alphabet';
 import type { Dawg } from '../dict/dawg';
@@ -35,6 +36,12 @@ export interface LocalMove {
   action: 'play' | 'pass' | 'exchange' | 'resign';
   dir?: Direction;
   tiles?: Placement[];
+  /** ActionPlay only: the main word's first-letter coordinate. */
+  mainRow?: number;
+  mainCol?: number;
+  /** ActionPlay only: the words formed as alphabet-index letter arrays — the main word first, then
+   *  the cross words. Decoded to glyphs when building the history's MoveRecord. */
+  words?: number[][];
   /** ActionExchange only: number of tiles swapped. */
   count?: number;
   /** ActionExchange only: the swapped tiles (alphabet-index bytes, BLANK_INDEX for a blank),
@@ -252,8 +259,31 @@ export class LocalGame {
     return generateMoves(this.dawg, this.board, this.rackOf(this.toMove), this.vrs, Both);
   }
 
+  /** evaluatePlay scores a candidate placement for the current position without committing it,
+   *  returning its legality (dictionary + connectivity), score, the words it forms (as alphabet-index
+   *  arrays, main first) and the inferred direction. Backs the local move preview. */
+  evaluatePlay(tiles: Placement[]): { legal: boolean; score: number; words: number[][]; dir: Direction } {
+    const dir = playDirection(this.board, this.vrs, this.dawg, tiles);
+    const res = validatePlay(this.board, this.vrs, this.dawg, dir, tiles);
+    if (!res.legal || !res.move) return { legal: false, score: 0, words: [], dir };
+    const m = res.move;
+    return { legal: true, score: m.score, words: [m.main.letters, ...m.cross.map((w) => w.letters)], dir };
+  }
+
+  /** dictionaryHas reports whether the word (alphabet-index letters) is in the game's dictionary. */
+  dictionaryHas(word: readonly number[]): boolean {
+    return this.dawg.indexOf(word) >= 0;
+  }
+
   // --- turns -----------------------------------------------------------------
 
+  /** submitPlay infers the play's orientation from the placement (like the server's SubmitPlay),
+   *  then plays it — the client submits tiles without a direction and the engine resolves it. */
+  submitPlay(tiles: Placement[]): LocalMove {
+    const dir = playDirection(this.board, this.vrs, this.dawg, tiles);
+    return this.play(dir, tiles);
+  }
+
   /** play validates and applies the current player's placement, scores it, refills the rack and
    *  advances the turn (or ends the game). Throws GameError on an illegal play. */
   play(dir: Direction, tiles: Placement[]): LocalMove {
@@ -277,6 +307,9 @@ export class LocalGame {
       action: 'play',
       dir,
       tiles: tiles.map((t) => ({ ...t })),
+      mainRow: move.main.row,
+      mainCol: move.main.col,
+      words: [move.main.letters, ...move.cross.map((w) => w.letters)],
       score: move.score,
       total: this.scores[player],
     };