fix(marker): per-link attribution via link field in MARK signals

A uBridge MARK signal carries only node= and filter= (no bridge/link field), so when one node is the capture side for several links that share a marker name (always the case for global-{name} definitions on a multi-interface node) the signals were indistinguishable and the (node, filter) registry collapsed them to a single link.

The mark filter is now stamped with its link id (mark <bpf> ... link <link_id>); uBridge echoes it verbatim (link=<link_id>) and the listener uses the signal's link= as the authoritative link_id of the marker.match event, falling back to the registry only for legacy signals without it. base_node and iou apply paths pass link_id; covered by two new listener tests.

gns3server/compute/iou/iou_vm.py

@@ -1137,6 +1137,11 @@ class IOUVM(BaseNode):
             )
             if tag is not None:
                 cmd += f" tag {tag}"
+            # IOU uses one per-node IOL-BRIDGE for every link, so bridge+filter
+            # are identical across this node's links — `link` is the only way the
+            # controller can tell their signals apart (contract §3.2).
+            if link_id:
+                cmd += f" link {link_id}"
             cmd += ' pcap "{path}"'.format(path=pcap_path)
             try:
                 await self._ubridge_send(cmd)