Add --dump-style-invalidation-counters=N to Ladybird and propagate it
to WebContent helper processes.
When enabled, WebContent dumps the current document style invalidation
counters with dbgln() after every N recorded style invalidations. This
makes it possible to collect the counters while browsing without adding
temporary C++ logging.
Node.cpp still contained the CSS policy for full style invalidation and
property-based invalidation plans. Move that logic into
CSS::Invalidation::NodeInvalidator.
Node remains the public DOM entry point for callers that need to
invalidate style. The helper now owns the :has() metadata probing,
style-scope plan lookup, and subtree/sibling invalidation scheduling.