Broad shadow-root stylesheet changes already restyle the whole shadow
tree, but host-side fallout does not always need a document-wide
invalidation. Split the host-side reach classification so selectors
contained to the host subtree, such as `:host *` and `:host > *`,
invalidate the host, while sibling-escaping selectors such as
`:host + :has(*)` still invalidate the host's root.
Recognize sibling escapes through positive selector-list pseudos such as
`:is()` and `:where()` as well, including selectors like
`:is(:host) + :has(*)` and `:is(:host + .item)`.
This avoids turning host-contained shadow stylesheet changes into full
document style invalidations. On https://pomax.github.io/bezierinfo/,
this reduces the time to produce a layout tree from about 8.7s to 3.6s
on my machine.
Avoid forcing a full style update when a connected inline <style> sheet
inserts an ordinary style rule. Build a targeted invalidation set from
the inserted rule and walk only the affected roots instead.
Introduce the shared StyleSheetInvalidation helper so later stylesheet
mutation paths can reuse the same selector analysis and root application
logic. It handles trailing-universal selectors, pseudo-element-only
rightmost compounds, and shadow-host escapes through ::slotted(...) and
:host combinators.
Keep the broad invalidate_owners() path for constructed stylesheets and
other sheet kinds whose TreeScope interactions still require it.
