feat(engine): overridePieceMoves hook

Phase A.4 of the rule-variants epic — adds the 'replace the default
move generator for this piece' extension point. First non-undefined
wins; later matches emit a dev-mode console.warn. The winning set
SKIPS default-generator + transformMoveGenerator + getExtraMoves +
en-passant/castling/promotion synthesis; filterMoves still composes
on top.

- Adds overridePieceMoves hook on PresetDef with full docs covering
  execution order, skipped downstream steps, filterMoves layering,
  and collision semantics.
- engine.getAllLegalMoves per-piece loop now checks overrides first;
  override path replaces moves entirely; default path unchanged when
  every preset returns undefined.
- Collision warning is dev-only (suppressed when NODE_ENV=production).
  First registered preset wins; second's return is ignored but
  console.warn names both preset ids.
- New override-piece-moves.test.ts (7 tests): default knight count
  baseline, lame-knight single-square override, undefined-declines
  fallthrough, collision warning fires with both preset ids in the
  message, production-mode suppression, filterMoves composes on top
  of overrides, unrelated piece types unaffected.

Tests: 1448 passing (was 1441, +7 new). Blocks: unblocks E.1
(berolina-pawns), E.2 (berolina-pawns-2).

.sisyphus/notepads/rule-variants/learnings.md

@@ -194,3 +194,53 @@ section).
 - `session.allFacts()` automatically carries the new fact across
   serialization boundaries (snapshot/restore) without protocol
   changes — no server-side work needed for the A.3 addition.
+
+## [2026-04-20 20:17] Task: A.4 — overridePieceMoves hook
+
+**Shipped**:
+- `overridePieceMoves` hook on `PresetDef`
+  (`packages/chess/src/presets/registry.ts`). Signature:
+  `(engine, pieceId) => readonly LegalMove[] | undefined`.
+- `engine.getAllLegalMoves` restructured for per-piece dispatch:
+  1. Poll every scoped preset's `overridePieceMoves`. First
+     non-undefined wins.
+  2. If override wins → replace moves entirely. SKIP: default
+     type-registry generator, `transformMoveGenerator` chain,
+     `getExtraMoves` for this piece, en-passant/castling/promotion
+     synthesis (preset owns those mechanics).
+  3. If no override → unchanged default path (type registry +
+     transforms + en-passant + castling + promotion +
+     `getExtraMoves`).
+  4. `filterMoves` runs REGARDLESS (layering stays useful).
+  5. Collision: second non-undefined return triggers
+     `console.warn` (dev only — suppressed when `NODE_ENV=production`).
+     First winner stands.
+- New test file: `override-piece-moves.test.ts` (7 tests). Covers:
+  baseline knight count unchanged, lame-knight prototype, undefined-
+  return fallthrough, collision warning fires, production-mode
+  suppression, `filterMoves` composes on top of overrides, unrelated
+  piece types unaffected.
+
+**Verification**: `bun run check` green. 1448 tests (was 1441, +7).
+
+**Gotchas**:
+- `engine.getPieceAt` is private. Test presets need occupancy
+  checks → use `engine.session.allFacts().find(f => f.attr === "Position" && f.value === sq)`.
+  Same friction every future preset test will hit. Flagged for
+  Phase A.5 docs consideration but leaving the private as-is per
+  "don't refactor during features".
+- Override collisions are a guardrail, not opt-in. The design
+  choice is "warn, don't throw" because the incompatibleWith graph
+  is the authoritative contract — crashing on collision would
+  make it harder to iteratively develop two competing overrides.
+
+## Phase A status at end of 20:17
+
+All 4 hooks landed on master:
+- A.1: 4d05473 — getRoyalPieces + engine royal dispatch
+- A.2: db8145f — filterLegalMoves
+- A.3: f9475e9 — shouldAdvanceTurn + HalfMovesThisTurn
+- A.4: (next commit) — overridePieceMoves
+
+Remaining in Phase A: A.5 — verification gate + PRESET-API.md
+hook-ordering doc + phase summary.