Cache lightweight selector insights on each stylesheet so cold style
scopes can answer whether :has() invalidation is relevant without
building a complete rule cache. This avoids forcing rule caches for
shadow roots whose active sheets do not contain :has() selectors.
Imported sheets contribute to their parent sheet's effective rules, so
an imported sheet load or CSSOM change also clears ancestor selector
insight caches.
Add test-only counters and regression coverage for cold shadow roots
with and without :has() selectors, plus delayed imported :has() rules.
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