Profile-neutral DOM trait.
LayoutDom is the ID-first surface that serval-layout (and other
read-only DOM walkers — reader-mode, serialization, querySelector helpers)
consume. It does not commit to a backing store: serval-static-dom's
StaticDocument and a future scripted-DOM provider both implement it.
Design rationale and prior art: see
docs/2026-05-16_layout_dom_api_design.md.