feat(engine): shouldAdvanceTurn hook + HalfMovesThisTurn fact

Phase A.3 of the rule-variants epic — preset-controlled turn flip
gating. Enables double-move (white plays 2, then black plays 2) and
monster (scope-aware: white plays 2, black plays 1) without engine
changes at the preset layer.

- Adds HalfMovesThisTurn: number to ChessAttrMap. Seeded to 0 in
  applyLayout. Distinct from HalfmoveClock (FIDE 50-move rule) —
  this counter is within-turn and resets on flip.
- Adds TurnAdvanceContext + shouldAdvanceTurn hook to PresetDef.
  Hook receives post-increment count via ctx.halfMovesThisTurn so
  'play N half-moves before flipping' reads as the predicate
  'halfMovesThisTurn < N -> false'.
- engine.applyMove: increments HalfMovesThisTurn BEFORE polling
  shouldAdvanceTurn; first false wins; on flip resets to 0 and
  increments FullmoveNumber after black; onTurnStart is gated on
  shouldAdvance (vetoed flips don't fire turn-start hooks, so
  mid-turn stamina regen bugs are prevented by construction).
- starting-position.test.ts updated with HalfMovesThisTurn=0 seed
  assertion.
- New turn-advance.test.ts (7 tests): seeding, baseline unchanged,
  never-flip veto, flip-after-two (double-move surface), onTurnStart
  gating across vetoed + non-vetoed flips, FullmoveNumber increments
  only on actual flips after black.

Tests: 1441 passing (was 1434, +7 new). Blocks: unblocks B.2
(double-move), B.3 (monster-rules), B.4 (first-promotion-wins).

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

@@ -153,3 +153,44 @@ section).
   would be a visible diff. Keeping enforcement out for now
   because the 1 call site per legal-move request is hot-path-
   adjacent and a `Set`-based prune would show up.
+
+## [2026-04-20 20:13] Task: A.3 — shouldAdvanceTurn hook + HalfMovesThisTurn fact
+
+**Shipped**:
+- `HalfMovesThisTurn: number` added to `ChessAttrMap`
+  (`packages/chess/src/schema.ts`). Seeded to 0 alongside `Turn` in
+  `applyLayout` (`packages/chess/src/starting-position.ts`).
+- `TurnAdvanceContext` type + `shouldAdvanceTurn` hook on
+  `PresetDef`. Hook receives post-increment count. First `false`
+  wins; iteration stops.
+- `engine.applyMove`:
+  1. Increments `HalfMovesThisTurn` BEFORE polling (so hooks see
+     count including the move just committed).
+  2. Polls every active preset's `shouldAdvanceTurn`; first `false`
+     cancels the flip.
+  3. On flip: updates `Turn`, resets `HalfMovesThisTurn` to 0,
+     increments `FullmoveNumber` after black.
+  4. On skip: leaves `Turn` and `HalfMovesThisTurn` alone (count
+     accumulates).
+  5. `onTurnStart` is GATED on `shouldAdvance === true`. Vetoed
+     flips don't fire turn-start (so double-move stamina doesn't
+     regen mid-turn).
+- `starting-position.test.ts` updated to assert the new fact seed.
+- New `turn-advance.test.ts` — 7 tests: seeding, baseline unchanged,
+  never-flip veto accumulates count, flip-after-two double-move
+  surface, `onTurnStart` gating against vetoes, FullmoveNumber only
+  on real flips.
+
+**Verification**: `bun run check` green. 1441 tests (was 1434, +7).
+
+**Gotchas**:
+- Two DISTINCT half-move counters now exist. `HalfmoveClock`
+  (FIDE 50-move rule) vs `HalfMovesThisTurn` (this epic's
+  within-turn counter). Documented in schema comment; hook docs
+  emphasize the distinction.
+- `tickAfterMove(color)` runs on the mover's color regardless of
+  flip decision — correct: a 3-turn buff ticks per actual move
+  made, not per Turn flip. Matches existing semantics.
+- `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.