Node.cpp still contained selector-specific policy for sibling and
same-parent-move structural invalidation. Move that logic into
CSS::Invalidation::StructuralMutationInvalidator so DOM mutation code
can delegate structural selector dependency handling.
This is a behavior-preserving extraction. It keeps the existing
previous-sibling walk guard, sibling-distance checks, shadow-root
marking, and ancestor child-needs-style propagation.
