diff --git a/packages/chess/src/engine.ts b/packages/chess/src/engine.ts
index dab6bf4..5a14fee 100644
--- a/packages/chess/src/engine.ts
+++ b/packages/chess/src/engine.ts
@@ -1385,17 +1385,41 @@ export class ChessEngine {
   }
 
   checkGameResult(): GameResult {
-    // Preset override path: run onCheckGameResult on every active preset
-    // in registration order. First non-undefined return wins. This lets
-    // capture-to-win and last-piece-standing redefine "game over"
-    // without touching engine internals.
+    // Preset override path: run onCheckGameResult on every active
+    // preset in registration order. Semantics:
+    //
+    //   - A TERMINAL return ("white-wins" | "black-wins" | "draw" |
+    //     "checkmate" | "stalemate" | "draw-*") immediately wins —
+    //     first terminal return locks the result.
+    //   - An "ongoing" return DOES NOT short-circuit — it's a
+    //     "suppress the engine's default checkmate/stalemate poll"
+    //     hint. Remembered so that if NO preset returns a terminal
+    //     result and no later preset downgrades, we skip the default
+    //     check/mate defaults and return "ongoing".
+    //   - `undefined` = no opinion, move on.
+    //
+    // This two-phase protocol is what makes presets like `piece-hp`
+    // (which suppresses default checkmate via "ongoing" because HP
+    // lets a king survive check) compose with presets like
+    // `first-promotion-wins` (which declares a winner mid-game
+    // before any check/mate default would fire). Both run; the
+    // terminal declaration wins.
     const resultCtx: GameResultHookContext = { engine: this };
+    let suppressDefaults = false;
     for (const entry of this.activePresets.list()) {
       const def = PRESET_REGISTRY.get(entry.id);
       const override = def?.onCheckGameResult?.(resultCtx);
-      if (override !== undefined) return override;
+      if (override === undefined) continue;
+      if (override === "ongoing") {
+        suppressDefaults = true;
+        continue;
+      }
+      // Any other GameResult is terminal — lock it in and stop.
+      return override;
     }
 
+    if (suppressDefaults) return "ongoing";
+
     const nextColor = this.getCurrentTurn();
     const nextRoyalIds = this.getActiveRoyalEntityIds(nextColor);
     if (isCheckmate(this.session, nextColor, nextRoyalIds)) return "checkmate";
diff --git a/packages/chess/src/presets/monster-rules.test.ts b/packages/chess/src/presets/monster-rules.test.ts
new file mode 100644
index 0000000..b85042a
--- /dev/null
+++ b/packages/chess/src/presets/monster-rules.test.ts
@@ -0,0 +1,368 @@
+/**
+ * Behavioural tests for the `monster-rules` preset (Phase B.3 of the
+ * rule-variants epic).
+ *
+ * Monster is an ASYMMETRIC flip preset: WHITE plays two half-moves
+ * per turn, BLACK plays one. It inspects `ctx.mover` rather than
+ * `ctx.scope` — the asymmetry is a rule property, not an activation
+ * property. See `monster-rules.ts` for the rationale.
+ *
+ * Coverage
+ *   - Preset registration + activation sanity.
+ *   - Asymmetric flip: W veto on move 1, W flip on move 2, B flip
+ *     on move 1. Turn / HalfMovesThisTurn / FullmoveNumber all
+ *     track correctly.
+ *   - onTurnStart cadence: 2 invocations over a W W B sequence
+ *     (white flip on move 2, black flip on move 1 = two flips).
+ *   - Composition with the canonical `monster` layout (king + 4
+ *     pawns vs. full black army).
+ *   - Checkmate delivered by white (on its flipping second
+ *     half-move) is detected the moment the flip lands.
+ *   - Checkmate delivered by black (on its single move) is
+ *     detected the moment the flip lands.
+ *   - Incompatibility with `double-move` is enforced at
+ *     `setActivePresets` time.
+ *   - Mid-turn state is reflected in `session.allFacts()` during
+ *     a vetoed W1.
+ */
+import { describe, it, expect } from "vitest";
+import "./index.js";
+import { ChessEngine } from "../engine.js";
+import { GAME_ENTITY } from "../schema.js";
+import { MONSTER_LAYOUT } from "../layouts/monster.js";
+import { PRESET_REGISTRY } from "./registry.js";
+import { clearBoard, placePiece } from "./test-utils.js";
+import { isInCheck } from "../rules/check.js";
+
+const MONSTER_RULES_ID = "monster-rules";
+
+function firstLegalMove(engine: ChessEngine) {
+  const moves = engine.getAllLegalMoves();
+  if (moves.length === 0) throw new Error("no legal moves available");
+  return moves[0]!;
+}
+
+function activate(engine: ChessEngine): void {
+  engine.setActivePresets([
+    { id: MONSTER_RULES_ID, scope: "both", turnsRemaining: null },
+  ]);
+}
+
+// ─────────────────────────────────────────────────────────────────────
+// Registration sanity
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — preset definition sanity", () => {
+  it("is registered with the expected id, name, and flip hook", () => {
+    const def = PRESET_REGISTRY.get(MONSTER_RULES_ID);
+    expect(def).toBeDefined();
+    expect(def!.id).toBe(MONSTER_RULES_ID);
+    expect(def!.name).toBe("Monster");
+    expect(def!.requires).toEqual([]);
+    expect(def!.incompatibleWith).toContain("double-move");
+    expect(def!.incompatibleWith).toContain("suicide-chess");
+    expect(def!.incompatibleWith).toContain("capture-all");
+    expect(typeof def!.shouldAdvanceTurn).toBe("function");
+  });
+
+  it("activates without throwing on a fresh engine (no layout coupling)", () => {
+    const engine = new ChessEngine();
+    expect(() => activate(engine)).not.toThrow();
+    const ids = engine.activePresets.list().map((e) => e.id);
+    expect(ids).toContain(MONSTER_RULES_ID);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// Core asymmetric cadence
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — asymmetric flip cadence", () => {
+  it("white's first move is vetoed: Turn stays white, count=1", () => {
+    const engine = new ChessEngine();
+    activate(engine);
+
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+    // FullmoveNumber only increments after black completes a turn.
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(1);
+  });
+
+  it("white's second move flips to black: count resets to 0", () => {
+    const engine = new ChessEngine();
+    activate(engine);
+
+    engine.applyMove(firstLegalMove(engine)); // W1 veto
+    engine.applyMove(firstLegalMove(engine)); // W2 flip
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+    // Still 1 — black hasn't completed their turn yet.
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(1);
+  });
+
+  it("black's single move flips to white: NO veto because mover is black", () => {
+    const engine = new ChessEngine();
+    activate(engine);
+
+    engine.applyMove(firstLegalMove(engine)); // W1 veto
+    engine.applyMove(firstLegalMove(engine)); // W2 flip → black
+    engine.applyMove(firstLegalMove(engine)); // B1 — default flip, NOT vetoed
+
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+    // After black completed a turn, fullmove increments.
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(2);
+  });
+
+  it("FullmoveNumber only increments after black's single-move turn (not after white's W2)", () => {
+    const engine = new ChessEngine();
+    activate(engine);
+
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(1);
+
+    engine.applyMove(firstLegalMove(engine)); // W1 veto
+    engine.applyMove(firstLegalMove(engine)); // W2 flip → black
+    // Flip to black does NOT increment — fullmove ticks only after black.
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(1);
+
+    engine.applyMove(firstLegalMove(engine)); // B1 flip → white, increment
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(2);
+
+    engine.applyMove(firstLegalMove(engine)); // W1 veto
+    engine.applyMove(firstLegalMove(engine)); // W2 flip → black, NO increment yet
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(2);
+
+    engine.applyMove(firstLegalMove(engine)); // B1 flip → white, increment
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(3);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// onTurnStart cadence
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — onTurnStart cadence", () => {
+  const PROBE_ID = "test-monster-rules-onturnstart-probe";
+
+  it("fires exactly twice over a full W W B sequence (2 flips)", () => {
+    let invocations = 0;
+    PRESET_REGISTRY.register({
+      id: PROBE_ID,
+      name: "Turn-start probe (monster-rules)",
+      description: "Counts onTurnStart invocations alongside monster-rules.",
+      incompatibleWith: [],
+      requires: [],
+      onTurnStart() {
+        invocations++;
+      },
+    });
+
+    const engine = new ChessEngine();
+    engine.setActivePresets([
+      { id: MONSTER_RULES_ID, scope: "both", turnsRemaining: null },
+      { id: PROBE_ID, scope: "both", turnsRemaining: null },
+    ]);
+
+    expect(invocations).toBe(0);
+    engine.applyMove(firstLegalMove(engine)); // W1 veto — no fire
+    expect(invocations).toBe(0);
+    engine.applyMove(firstLegalMove(engine)); // W2 flip — fire
+    expect(invocations).toBe(1);
+    engine.applyMove(firstLegalMove(engine)); // B1 flip — fire
+    expect(invocations).toBe(2);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// Monster layout composition
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — composition with monster layout", () => {
+  it("plays a W W B W W B sequence without error (king + 4 pawns vs. full army)", () => {
+    const engine = new ChessEngine({ layout: MONSTER_LAYOUT });
+    activate(engine);
+
+    // Sanity: layout seeded as documented (king e1, four pawns on rank 2).
+    const facts = engine.session.allFacts();
+    const whitePieceCount = facts.filter(
+      (f) =>
+        f.attr === "Color" &&
+        f.value === "white" &&
+        (f.id as number) > 0,
+    ).length;
+    // 1 king + 4 pawns.
+    expect(whitePieceCount).toBe(5);
+
+    // Play 6 half-moves: W W B W W B.
+    engine.applyMove(firstLegalMove(engine)); // W1 veto
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+
+    engine.applyMove(firstLegalMove(engine)); // W2 flip
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+
+    engine.applyMove(firstLegalMove(engine)); // B1 flip
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(2);
+
+    engine.applyMove(firstLegalMove(engine)); // W1 veto (turn 2)
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+
+    engine.applyMove(firstLegalMove(engine)); // W2 flip
+    expect(engine.getCurrentTurn()).toBe("black");
+
+    engine.applyMove(firstLegalMove(engine)); // B1 flip
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(3);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// Checkmate detection
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — checkmate detection", () => {
+  it("white delivers mate on W2 (the flipping half-move): detected immediately after flip", () => {
+    // Scholar's-mate adaptation under monster cadence:
+    //   W1 e4 (veto), W2 Bc4 (flip to black)
+    //   B1 a6 (flip to white)
+    //   W1 Qh5 (veto) — not check yet
+    //   W2 Qxf7# (flip to black) — mate.
+    const engine = new ChessEngine();
+    activate(engine);
+
+    const play = (fromSq: number, toSq: number) => {
+      const mv = engine.findMove(fromSq, toSq);
+      expect(mv, `legal move ${fromSq}->${toSq}`).not.toBeNull();
+      engine.applyMove(mv!);
+    };
+    // Algebraic squares a1=0 … h8=63 (file + rank*8). Use imports for clarity.
+    const sq = (file: string, rank: number) =>
+      "abcdefgh".indexOf(file) + (rank - 1) * 8;
+
+    play(sq("e", 2), sq("e", 4)); // W1 veto
+    play(sq("f", 1), sq("c", 4)); // W2 flip to black
+    expect(engine.getCurrentTurn()).toBe("black");
+
+    play(sq("a", 7), sq("a", 6)); // B1 flip to white
+    expect(engine.getCurrentTurn()).toBe("white");
+
+    play(sq("d", 1), sq("h", 5)); // W1 veto
+    expect(engine.getCurrentTurn()).toBe("white");
+
+    const mateMove = engine.findMove(sq("h", 5), sq("f", 7));
+    expect(mateMove, "Qxf7 must be legal as W2").not.toBeNull();
+    const result = engine.applyMove(mateMove!); // W2 flip + mate
+    expect(result).toBe("checkmate");
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(engine.checkGameResult()).toBe("checkmate");
+  });
+
+  it("white checking black on W1: Turn stays white, black is in check at session level, W2 still available", () => {
+    // On a crafted back-rank position, W1 delivers rook check; veto
+    // keeps Turn on white; W2 plays a legal harmless move that
+    // preserves the mating pattern; flip to black; mate detected.
+    const engine = new ChessEngine();
+    clearBoard(engine, { preserveKings: false });
+    placePiece(engine, "king", "white", "g1");
+    placePiece(engine, "king", "black", "h8");
+    placePiece(engine, "pawn", "black", "g7");
+    placePiece(engine, "pawn", "black", "h7");
+    placePiece(engine, "rook", "white", "a1");
+    placePiece(engine, "knight", "white", "b1");
+    // Default Turn is "white" (seeded by applyLayout).
+
+    activate(engine);
+
+    // W1: Ra1→a8 delivers back-rank check/mate against black king h8.
+    const sq = (file: string, rank: number) =>
+      "abcdefgh".indexOf(file) + (rank - 1) * 8;
+    const check = engine.findMove(sq("a", 1), sq("a", 8));
+    expect(check, "Ra1-a8 must be legal").not.toBeNull();
+    const mid = engine.applyMove(check!);
+
+    // Veto: Turn stays white, count=1. checkGameResult checks white
+    // (side-to-move under veto), which isn't in check, so "ongoing".
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+    expect(mid).toBe("ongoing");
+    // Black IS in check at the pure-session level.
+    expect(isInCheck(engine.session, "black")).toBe(true);
+
+    // W2: Nb1→c3 (harmless). After flip, black is still mated.
+    const harmless = engine.findMove(sq("b", 1), sq("c", 3));
+    expect(harmless, "Nb1-c3 must be legal as W2").not.toBeNull();
+    const final = engine.applyMove(harmless!);
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(final).toBe("checkmate");
+    expect(engine.checkGameResult()).toBe("checkmate");
+  });
+
+  it("black delivers mate on its single move: detected immediately after flip", () => {
+    // Reverse back-rank: black plays B1 Ra8-a1# against white king h1.
+    const engine = new ChessEngine();
+    clearBoard(engine, { preserveKings: false });
+    placePiece(engine, "king", "white", "h1");
+    placePiece(engine, "king", "black", "g8");
+    placePiece(engine, "pawn", "white", "g2");
+    placePiece(engine, "pawn", "white", "h2");
+    placePiece(engine, "rook", "black", "a8");
+
+    // Force black-to-move so a single applyMove from black is B1.
+    engine.session.insert(GAME_ENTITY, "Turn", "black");
+
+    activate(engine);
+
+    const sq = (file: string, rank: number) =>
+      "abcdefgh".indexOf(file) + (rank - 1) * 8;
+    const mate = engine.findMove(sq("a", 8), sq("a", 1));
+    expect(mate, "Ra8-a1 must be legal as B1").not.toBeNull();
+    const result = engine.applyMove(mate!); // B1 flips to white under monster
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(result).toBe("checkmate");
+    expect(engine.checkGameResult()).toBe("checkmate");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// Incompatibility enforcement
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — incompatibility", () => {
+  it("throws when activated alongside double-move under overlapping scope", () => {
+    const engine = new ChessEngine();
+    expect(() =>
+      engine.setActivePresets([
+        { id: MONSTER_RULES_ID, scope: "both", turnsRemaining: null },
+        { id: "double-move", scope: "both", turnsRemaining: null },
+      ]),
+    ).toThrow(/incompatible/i);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// Session snapshot integrity
+// ─────────────────────────────────────────────────────────────────────
+
+describe("monster-rules — session snapshot integrity", () => {
+  it("HalfMovesThisTurn appears in allFacts() during a vetoed W1 state", () => {
+    const engine = new ChessEngine();
+    activate(engine);
+    engine.applyMove(firstLegalMove(engine)); // W1 veto
+
+    const facts = engine.session.allFacts();
+    const halfMoveFact = facts.find(
+      (f) => f.id === GAME_ENTITY && f.attr === "HalfMovesThisTurn",
+    );
+    expect(halfMoveFact).toBeDefined();
+    expect(halfMoveFact!.value).toBe(1);
+
+    const turnFact = facts.find(
+      (f) => f.id === GAME_ENTITY && f.attr === "Turn",
+    );
+    expect(turnFact?.value).toBe("white");
+  });
+});
diff --git a/packages/chess/src/presets/monster-rules.ts b/packages/chess/src/presets/monster-rules.ts
new file mode 100644
index 0000000..415793c
--- /dev/null
+++ b/packages/chess/src/presets/monster-rules.ts
@@ -0,0 +1,74 @@
+/**
+ * Preset: Monster Chess rules (rule-variants epic, Phase B.3).
+ *
+ * Asymmetric double-move variant. WHITE plays two half-moves per
+ * turn; BLACK plays one. Canonically paired with the `monster`
+ * layout (see `layouts/monster.ts`), which gives white a reduced
+ * army (king + 4 pawns) — the "play twice" compensation is what
+ * keeps the variant balanced at high level.
+ *
+ * How the veto works
+ * ──────────────────
+ * The engine invokes every active preset's `shouldAdvanceTurn`
+ * hook AFTER committing the move and AFTER incrementing
+ * `HalfMovesThisTurn`. The hook sees the post-increment count and
+ * the color of the mover. Phase A.3's semantics are "first false
+ * wins": as soon as any preset vetoes, the engine skips the flip
+ * (and `HalfMovesThisTurn` is NOT reset), so the mover plays
+ * again.
+ *
+ * Monster's rule:
+ *   - If `mover === "white"` AND `halfMovesThisTurn < 2` → veto
+ *     (return `false`). White gets a second half-move.
+ *   - Otherwise return `undefined` (no opinion) — the engine's
+ *     default flips the turn. In particular BLACK's single move
+ *     always flips by default.
+ *
+ * Why scope inspection isn't needed here
+ * ─────────────────────────────────────
+ * Unlike some scope-aware presets that read `engine.activePresets`
+ * to decide behaviour, this preset decides purely from `ctx.mover`.
+ * That's intentional: Monster's rule is "white has a mandatory
+ * double-move", full stop — it does NOT depend on whether the
+ * preset was activated with `scope: "white"` vs `scope: "both"`.
+ * (The scope still governs duration-ticking and preset-lifecycle
+ * semantics per `ActivePresetSet`, but not the hook's behaviour.)
+ *
+ * Incompatibilities
+ * ─────────────────
+ * Declared incompatible with:
+ *   - `double-move` — that preset ALSO vetoes the flip after 1,
+ *     so stacking them either double-vetoes or produces undefined
+ *     orderings. Users should pick one.
+ *   - `suicide-chess`, `capture-all` — these redefine the notion
+ *     of "legal move" in ways that interact poorly with a
+ *     mandatory extra half-move per turn (e.g. white's first move
+ *     might be forced into a suicide capture, leaving no sensible
+ *     second move).
+ *
+ * Checkmate / stalemate detection is unaffected: the engine
+ * continues to poll its default checkmate/stalemate predicates
+ * against the color that is NEXT TO MOVE after each flip. With
+ * monster-rules, a mate delivered by white's first half-move
+ * surfaces as "checkmate" once white's second half-move lands
+ * and the flip actually proceeds. A mate delivered by black on
+ * its single move surfaces immediately on flip, same as default.
+ */
+import { PRESET_REGISTRY } from "./registry.js";
+
+PRESET_REGISTRY.register({
+  id: "monster-rules",
+  name: "Monster",
+  description:
+    "White plays two half-moves per turn; black plays one. Classic Monster chess pairing.",
+  incompatibleWith: ["double-move", "suicide-chess", "capture-all"],
+  requires: [],
+  shouldAdvanceTurn({ mover, halfMovesThisTurn }) {
+    // White gets a mandatory second half-move. The engine polls this
+    // hook AFTER incrementing HalfMovesThisTurn, so seeing 1 means
+    // "white just played its first move this turn" — veto.
+    if (mover === "white" && halfMovesThisTurn < 2) return false;
+    // Black, or white's second half-move: let the default flip.
+    return undefined;
+  },
+});
diff --git a/packages/chess/src/presets/registry.ts b/packages/chess/src/presets/registry.ts
index cd4f6b5..e7dc15b 100644
--- a/packages/chess/src/presets/registry.ts
+++ b/packages/chess/src/presets/registry.ts
@@ -477,15 +477,36 @@ export interface PresetDef {
   readonly onAfterMove?: (ctx: MoveHookContext) => void;
 
   /**
-   * Hook into terminal-position detection. Return a concrete `GameResult`
-   * to OVERRIDE the engine's default checkmate/stalemate/draw logic;
-   * return `undefined` to let the default run. Multiple presets may
-   * register; the first one returning a non-undefined value wins. Order
-   * follows registration order (see PRESET_REGISTRY.getAll()).
+   * Hook into terminal-position detection.
    *
-   * Used by `capture-to-win` (first capture sets the winner) and
-   * `last-piece-standing` (annihilation replaces checkmate), which both
-   * redefine "when is the game over".
+   * Return semantics (the engine implements a two-phase protocol so
+   * presets compose rather than short-circuit each other):
+   *
+   *  - A TERMINAL `GameResult` (`"white-wins"`, `"black-wins"`,
+   *    `"draw"`, `"checkmate"`, `"stalemate"`, `"draw-50"`,
+   *    `"draw-3fold"`, `"draw-insufficient"`) immediately wins —
+   *    the engine stops polling and returns that value. First
+   *    terminal return locks the outcome.
+   *
+   *  - `"ongoing"` is a soft signal: it tells the engine "don't
+   *    run the default checkmate/stalemate/draw predicates when
+   *    nobody else declares terminal". The poll CONTINUES — a
+   *    later preset can still declare a winner. Use this when your
+   *    preset wants to suppress the default rules (e.g. `piece-hp`
+   *    suppresses default checkmate because an HP king can survive
+   *    attacks) without preventing other presets from declaring
+   *    terminal.
+   *
+   *  - `undefined` = no opinion; poll continues.
+   *
+   * Registration order: presets are polled in
+   * `engine.activePresets.list()` order (activation order).
+   *
+   * Used by `capture-to-win` (first capture sets the winner),
+   * `last-piece-standing` (annihilation replaces checkmate),
+   * `first-promotion-wins` (first pawn promotion wins), and
+   * `piece-hp` (returns `"ongoing"` to suppress default mate while
+   * HP kings can survive).
    */
   readonly onCheckGameResult?: (ctx: GameResultHookContext) => GameResult | undefined;