marker: fine-grained filter ops, clean pcap on remove

Deleting or updating a marker no longer triggers a full NIO reapply
(reset_packet_filters + re-add), which closed/reopened every sibling
marker's pcap via uBridge. Instead operate on single filters:

- stop_marker: bridge delete_packet_filter + unlink the pcap (works with
  the node stopped; filter removal is skipped, the file is still deleted).
- update_marker: bpf/tag/direction → rebuild just that filter (delete + add);
  enabled → instant toggle; color/highlight_duration → stored only.
- compute delete_marker_capture / rebuild_marker_filter + per-node routes
  (DELETE /markers/{name}, PUT /markers/{name}/rebuild) + MarkerRebuild schema.

IOU overrides _ubridge_delete_marker_filter for iol_bridge; rebuild reuses
the already-overridden add/delete/set, so IOU needs no rebuild override.

gns3server/compute/iou/iou_vm.py

@@ -1340,6 +1340,17 @@ class IOUVM(BaseNode):
             if n == name:
                 await self._ubridge_send(f"iol_bridge enable_packet_filter {location} {name} {state}")
 
+    async def _ubridge_delete_marker_filter(self, location, name):
+        """IOU override: remove a single marker filter via ``iol_bridge``
+        (location = ``{bridge} {bay} {unit}``), not a bridge-wide reset."""
+
+        if not (self._ubridge_hypervisor and self._ubridge_hypervisor.is_running()):
+            return
+        try:
+            await self._ubridge_send(f"iol_bridge delete_packet_filter {location} {name}")
+        except UbridgeError as e:
+            log.warning("Could not remove marker filter '%s' from %s: %s", name, location, e)
+
     async def adapter_remove_nio_binding(self, adapter_number, port_number):
         """
         Removes an adapter NIO binding.