Overlay stack

Each owner document has an on-demand overlay stack stored in a WeakMap. No document is accessed during import. The topmost entry alone handles Escape. Opening or closing synchronizes parent topmost metadata.

Outside pointer and focus logic treats explicit branches and higher stacked overlays as inside the parent. This prevents a nested menu, popover, or dialog from dismissing its ancestor. Modal scroll locks are reference-counted and sibling inert state is restored exactly.

Binding cleanup removes the stack entry even if the controller remains alive; destruction also releases the active binding.