fix: bypass CodeMirror posAtCoords for accurate cursor at non-100% CSS zoom

CSS zoom on document.documentElement causes a coordinate space mismatch
between mouse event clientX/Y (viewport space) and Range.getClientRects()
(CSS space), breaking CodeMirror's click-to-position mapping. The previous
requestMeasure() fix only recalibrated cached geometry, not this mismatch.

New approach: zoomCursorFix extension patches posAtCoords/posAndSideAtCoords
on the EditorView instance to use document.caretRangeFromPoint() — the
browser's native, zoom-aware API — with coord-adjustment fallback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useCodeMirror.test.ts

@@ -66,4 +66,14 @@ describe('useCodeMirror', () => {
     expect(spy).not.toHaveBeenCalled()
     spy.mockRestore()
   })
+
+  it('installs zoomCursorFix that overrides posAtCoords on the view instance', () => {
+    const ref = { current: container }
+    const { result } = renderHook(() =>
+      useCodeMirror(ref, 'hello world', false, noopCallbacks),
+    )
+    const view = result.current.current!
+    // The extension overrides posAtCoords on the instance (not the prototype)
+    expect(Object.prototype.hasOwnProperty.call(view, 'posAtCoords')).toBe(true)
+  })
 })