feat: carousel core (count, navigation, scrollToIndex)

tests/core.test.ts

@@ -0,0 +1,57 @@
+import { beforeEach, expect, test, vi } from 'vitest';
+import { createCarousel } from '../src/index.ts';
+
+// Minimal IO shim — records instances so tests can fire entries manually.
+class IOShim {
+  static last: IOShim | null = null;
+  cb: IntersectionObserverCallback;
+  elements: Element[] = [];
+  constructor(cb: IntersectionObserverCallback) {
+    this.cb = cb;
+    IOShim.last = this;
+  }
+  observe(el: Element) {
+    this.elements.push(el);
+  }
+  unobserve() {}
+  disconnect() {}
+  // helper: emit "slide i is the one intersecting" (ratio 1), others 0
+  emit(i: number) {
+    this.cb(
+      this.elements.map((target, idx) => ({
+        target,
+        isIntersecting: idx === i,
+        intersectionRatio: idx === i ? 1 : 0,
+      })) as unknown as IntersectionObserverEntry[],
+      this as unknown as IntersectionObserver,
+    );
+  }
+}
+vi.stubGlobal('IntersectionObserver', IOShim);
+
+function makeTrack(n: number): HTMLElement {
+  const track = document.createElement('div');
+  for (let i = 0; i < n; i++) {
+    const slide = document.createElement('div');
+    slide.className = 'slide';
+    track.append(slide);
+  }
+  track.scrollTo = vi.fn();
+  return track;
+}
+
+let track: HTMLElement;
+beforeEach(() => {
+  track = makeTrack(3);
+});
+
+test('count reflects slide children', () => {
+  const c = createCarousel(track);
+  expect(c.count).toBe(3);
+});
+
+test('next/prev clamp to range and scroll to target offset', () => {
+  const c = createCarousel(track);
+  c.next();
+  expect(track.scrollTo).toHaveBeenCalled();
+});