Mark elements reached by stepping through sibling combinators inside
:has() and use that breadcrumb during generic invalidation walks.
Keep the existing conservative sibling scans for mutations outside
those marked subtrees so nested :is(), :not(), and nesting cases
continue to invalidate correctly.
Also keep :has() eager within compounds that contain ::part(). Those
selectors retarget the remaining simple selectors to the part host, so
deferring :has() there changes which element the pseudo-class runs
against and can make ::part(foo):has(.match) spuriously match.
Add a counter-based sibling-scan test and a regression test covering
the ::part()/ :has() selector orderings.
