feat(renderer): useScrollSync hook (60fps throttling, ratio calc)

tests/unit/hooks/use-scroll-sync.test.ts

@@ -0,0 +1,26 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { renderHook, act } from '@testing-library/react';
+import { useScrollSync } from '@/hooks/use-scroll-sync';
+
+describe('useScrollSync', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    // Mock performance.now() to return 0 and not auto-advance
+    let time = 0;
+    vi.spyOn(performance, 'now').mockImplementation(() => time);
+  });
+
+  it('throttles editor scroll events to 60fps', () => {
+    const onScroll = vi.fn();
+    const { result } = renderHook(() => useScrollSync({ onEditorScroll: onScroll }));
+    const mockEvt = { currentTarget: { scrollTop: 100, scrollHeight: 1000, clientHeight: 200 } } as any;
+    // All calls within the same act() - performance.now() stays at 0
+    // First call passes, subsequent calls within FRAME_MS are throttled
+    act(() => {
+      result.current.handleEditorScroll(mockEvt);
+      result.current.handleEditorScroll(mockEvt);
+      result.current.handleEditorScroll(mockEvt);
+    });
+    expect(onScroll).toHaveBeenCalledTimes(1);
+  });
+});