fix(useElementByPoint): avoid a re-render on every frame in multiple mode

[ostapondo]

Description

With multiple: true, useElementByPoint re-renders the consuming component on
every animation frame, even when the pointer hasn't moved and the elements under
it haven't changed.

elementsFromPoint() returns a freshly allocated array on each call, so the rAF
loop hands setElement a new reference every frame. React compares by identity,
sees a change, and re-renders — roughly 60 times a second, for as long as the
hook is active. The single-element path doesn't have this problem, since
elementFromPoint() returns the same node and React bails out on its own.

The fix compares the hit list by contents and keeps the previous array when
nothing changed. I used the functional form of setElement so the comparison
sees the committed value without needing an extra ref.

Type of Change

• Bug fix
• New hook
• Enhancement to existing hook
• Documentation update
• Other (please describe)

Checklist

• I have read the Contributing Guide
• My code follows the project's coding style
• I have added tests for my changes
• All existing tests pass
• I have updated the documentation

No doc change — public API and return values are unchanged.

This hook had no spec file, so I added one. Three tests: the element is reported
for the single case, an unchanged hit list no longer re-renders (3 renders over 3
frames on main, 0 here), and a hit list that genuinely changes still updates —
that last one matters, since a comparison like this is exactly the kind of change
that can silently freeze state.