The reconcile pass in _ubridge_apply_markers walked the node-wide
_marker_filter_bridges map but compared against `desired`, which only
carries the markers of the NIO being updated. Updating any one link
therefore deleted every other link's markers (and their pcaps) on that
node — a regression from the add-only→reconcile switch. IOU's override
had the same flaw across its ports.
Guard the delete pass with the current bridge (base_node) / IOL location
(IOU) so only markers on the NIO being reconciled can be removed. Added
a regression test that fails without the guard.