Keep animated ImageStyleValue frame advancement owned by the
style value. The current frame and loop state live there, so a
separate document scheduler would duplicate ownership of that state.
Start the ImageStyleValue timer only while it has layout clients.
Stop it when the last client unregisters, or when a finite animation
completes. Expose a document-scoped active timer count through
internals for focused regression tests.
Clear image observers when layout nodes detach. Use current-node
cleanup for per-DOM-node clearing, and explicit subtree cleanup for
tree replacement, full tree teardown, and synthetic pseudo-elements.
This keeps large document clearing linear.
Unregister generated-content image providers during layout detach
instead of waiting for GC to finalize the provider.
Cover hidden animated background images, generated content images,
layout node replacement, full layout tree teardown, and document
scoping for the internals counter.
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