Nested navigables were represented through compositor surface ids owned
by the parent context. That forced CompositorState and ContextState to
maintain bidirectional attach/detach bookkeeping, publish child
snapshots into a surface map, and keep presentation mode variants just
to distinguish UI presentation from parent composition.
Record the child compositor context id directly in the display list and
let the compositor resolve it against the painting parent at playback
time. Child contexts now keep their parent context id and latest
rendered surface, while parents no longer track child maps or compositor
surface ids. UI presentation is represented separately from parent
composition, so closing a page only stops client presentation and nested
contexts keep using set_parent_context.
The compositor already used a GPU surface with bitmap readback when a
Vulkan Skia context existed but no native shared surface path was
available. The new Windows Direct3D backend has the same constraint for
now: it can render with Skia on the GPU, while publication still uses
shareable bitmaps.
CompositorState treats presence in m_contexts as the lifetime state
for a compositor context. create_context() creates the entry and
destroy_context() removes it, so ContextState::is_registered duplicated
the map membership invariant.
The top-level-traversable bit was also duplicate information. The
only compositor use was backing-store padding while a window resize is
in progress, and page-presenting contexts are already identified by
PagePresentationRegistration::Yes. Normalize resize-in-progress to No
for non-page-presenting contexts, then remove the flag from
CompositorState, the backing-store API, and the compositor IPC boundary.
