Two issues prevented slotted elements from correctly inheriting
styles from their assigned slot:
1. Element::element_to_inherit_style_from() was skipping the slot
element and returning the shadow host instead. This meant slotted
elements inherited from the host, completely ignoring any styles
on the slot itself.
2. When a slot element's style changed during the style tree walk,
its assigned (slotted) nodes were never marked for recomputation.
The tree walk follows the DOM tree, but slotted elements are DOM
children of the shadow host, not the slot, so they were missed.
Fix (1) by returning the slot directly as the inheritance parent.
Fix (2) by marking assigned nodes dirty in update_style_recursively
when a slot's style changes.
Cover various scenarios: element's own style change, ancestor style
change affecting inheritance, sibling changes not affecting our element,
shadow DOM elements, pseudo-elements, and repeated reads.
Also documents a pre-existing bug where slotted elements don't pick up
inherited style changes from their assigned slot.
