Adds the host-side hook for the forthcoming tart-guest-agent DragAndDrop RPC
that will perform a real drag-and-drop at the cursor's position inside the
guest instead of dumping files into a generic share folder.
- DropGeometry.normalize: pure helper turning a view-local drop point into
top-left (0..1) coordinates the guest agent can map onto its screen.
- VMContainerView.performDragOperation: capture draggingLocation on the
main thread (only valid here), compute normalized point, then dispatch
the copy off-main as before.
- synthesizeGuestDrop: documented no-op stub called from the copy completion
path. When the agent RPC lands, this becomes the real call site; until
then it returns false and the existing share-folder copy is the
user-visible result.
No behavior change for users today.
Dal Rupnik