_ubridge_apply_markers now installs only markers not already on the bridge
(uBridge's reset_packet_filters preserves mark filters), so an NIO update no
longer re-adds — and reopens — sibling markers' pcaps. _stop_ubridge clears
_marker_filter_bridges so a node restart re-installs everything (the map would
otherwise keep stale entries pointing at a fresh, empty uBridge).