feat: autoplay() module with hover/focus pause and reduce-motion guard

tests/autoplay.test.ts

@@ -0,0 +1,73 @@
+import { afterEach, beforeEach, expect, test, vi } from 'vitest';
+import { autoplay } from '../src/autoplay.ts';
+
+function fakeCarousel(count = 3) {
+  let index = 0;
+  return {
+    get index() {
+      return index;
+    },
+    count,
+    next: vi.fn(() => {
+      index = (index + 1) % count;
+    }),
+    prev: vi.fn(),
+    scrollToIndex: vi.fn(),
+    on: () => () => {},
+    destroy: vi.fn(),
+  };
+}
+
+// jsdom has no matchMedia — stub it. reduce = whether reduce-motion matches.
+function stubMatchMedia(reduce: boolean) {
+  vi.stubGlobal('matchMedia', (q: string) => ({
+    matches: reduce && q.includes('reduce'),
+    media: q,
+    addEventListener() {},
+    removeEventListener() {},
+  }));
+}
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  stubMatchMedia(false);
+});
+afterEach(() => {
+  vi.useRealTimers();
+  vi.unstubAllGlobals();
+});
+
+test('advances every interval', () => {
+  const c = fakeCarousel();
+  autoplay(c as never, { interval: 1000, root: document.createElement('div') });
+  vi.advanceTimersByTime(2000);
+  expect(c.next).toHaveBeenCalledTimes(2);
+});
+
+test('pause on pointerenter, resume on pointerleave', () => {
+  const c = fakeCarousel();
+  const root = document.createElement('div');
+  autoplay(c as never, { interval: 1000, root });
+  root.dispatchEvent(new Event('pointerenter'));
+  vi.advanceTimersByTime(3000);
+  expect(c.next).not.toHaveBeenCalled();
+  root.dispatchEvent(new Event('pointerleave'));
+  vi.advanceTimersByTime(1000);
+  expect(c.next).toHaveBeenCalledTimes(1);
+});
+
+test('does not start under prefers-reduced-motion', () => {
+  stubMatchMedia(true);
+  const c = fakeCarousel();
+  autoplay(c as never, { interval: 1000, root: document.createElement('div') });
+  vi.advanceTimersByTime(5000);
+  expect(c.next).not.toHaveBeenCalled();
+});
+
+test('stop() halts and removes listeners', () => {
+  const c = fakeCarousel();
+  const stop = autoplay(c as never, { interval: 1000, root: document.createElement('div') });
+  stop();
+  vi.advanceTimersByTime(5000);
+  expect(c.next).not.toHaveBeenCalled();
+});