Make style updates reach a fixed point when slot invalidation dirties
assigned nodes after their traversal point. During inherited-style
cascades, only the topmost changed element scans for descendant slots.
Animation inherited-style updates now include the target slot and walk
shadow-including descendants, so host animations propagate inherited
values through shadow trees and assigned slottables. Animated inherited
longhands also carry the same inherited-style invalidation signal as
regular style changes.
Mark custom elements dirty when their :defined state flips, so upgraded
elements do not keep stale :not(:defined) computed style. Add coverage
for slotted menu invalidation, descendant-slot scan counts, target slot
animations, host shadow-tree animations, and explicit inherit from an
animated non-inherited longhand.
Document.cpp still knew how style changes on a slot propagate to
assigned light-DOM nodes. Move that flat-tree inheritance invalidation
into CSS::Invalidation::SlotInvalidator.
The style update walk continues to decide when an element's style
changed. The helper now owns the ::slotted() consequence of dirtying
assigned slottables for a changed slot.
Slottable.cpp still handled the style invalidation fallout from slot
assignment changes directly. Move that ::slotted()-related policy into
CSS::Invalidation::SlotInvalidator.
Slot assignment remains DOM bookkeeping. The helper now owns the
choice to dirty element slottables when they gain or lose assignment
to a slot.
