feat(useMeasure): support tracking an element via options.ref

[TT-Wang]

What

Adds an optional options.ref parameter to useMeasure. When provided, the hook observes ref.current instead of the element assigned to the callback ref it returns.

const myRef = useRef(null);
const { width, height } = useMeasure({ ref: myRef })[1];
return <div ref={myRef}>...</div>;

Why

Closes #1227. This has been requested by multiple users in that thread:

• The returned callback ref is not a real useRef (no ref.current), so it cannot be combined with an existing ref.
• Without this, an element that already has a ref cannot be measured.
• It enables multiple useMeasure hooks in one component to track different elements.

The API is fully backward compatible: when options.ref is omitted, behavior is unchanged.

Notes

• When ref.current changes, the layout effect re-runs and observes the new element (covered by a test).
• Updated docs/useMeasure.md with the new usage.
• All 7 tests in tests/useMeasure.test.ts pass, including 2 new ones covering the external-ref path.