feat(engine): add transformMoveGenerator + modifyMoveAttrs preset hooks

Adds two optional hooks to PresetDef:

- transformMoveGenerator wraps a piece's move generator. Presets compose in list order; each receives the previous wrapper's output. Returned moves are pseudo-legal and still pass through the engine's self-check filter.

- modifyMoveAttrs lets presets contribute additive range/direction deltas for generators that support them.

ChessEngine.getAllLegalMoves folds the transform chain before getExtraMoves/filterMoves, preserving existing hook ordering and the downstream self-check filter.

Tests cover: no-op wrap preserves moves, wrap adds pawn backward, two wraps compose, and self-check filter still prunes transform-added illegal moves.

packages/chess/src/engine.ts

@@ -718,8 +718,28 @@ export class ChessEngine {
       .filter(p => p.type !== undefined);
 
     for (const piece of pieces) {
-      const getter = lookupMoveGenerator(piece.type);
-      if (!getter) continue;
+      const baseGetter = lookupMoveGenerator(piece.type);
+      // Fold the transformMoveGenerator chain over all active presets
+      // (scope-filtered for `color`). Each preset's wrapper receives the
+      // OUTPUT of the previous — composing cleanly. We seed with the
+      // type-registry generator, or a degenerate empty-move generator if
+      // the piece type is unknown (keeps later wrappers well-defined).
+      const scopedPresets = this.activePresets.getForColor(color);
+      const seedGetter: MoveGetter =
+        baseGetter ?? ((_s: Session, _id: EntityId) => [] as LegalMove[]);
+      const getter: MoveGetter = scopedPresets
+        .filter(p => p.transformMoveGenerator)
+        .reduce<MoveGetter>(
+          (gen, preset) => preset.transformMoveGenerator!(this, piece.id, gen),
+          seedGetter,
+        );
+      // Skip pieces with no known type AND no transform — same semantics
+      // as the original "unknown type ⇒ immobile" guard, but now a
+      // transform preset can still produce moves for an otherwise unknown
+      // type.
+      if (!baseGetter && scopedPresets.every(p => !p.transformMoveGenerator)) {
+        continue;
+      }
 
       let pieceMoves = getter(this.session, piece.id);
 
@@ -759,7 +779,7 @@ export class ChessEngine {
       // Order matters — `getExtraMoves` contributes to the set that
       // `filterMoves` operates on, so every active preset sees the full
       // aggregated set (including prior presets' additions).
-      const activePresets = this.activePresets.getForColor(color);
+      const activePresets = scopedPresets;
       for (const preset of activePresets) {
         if (preset.getExtraMoves) {
           pieceMoves = [