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/base_node.py

@@ -1073,7 +1073,7 @@ class BaseNode:
                 )
                 i += 1
 
-    async def _ubridge_add_marker_filter(self, bridge_name, name, bpf, pcap_path, tag=None):
+    async def _ubridge_add_marker_filter(self, bridge_name, name, bpf, pcap_path, tag=None, link_id=None):
         """
         Attach a `mark` packet filter to a uBridge bridge for traffic insight.
 
@@ -1104,6 +1104,11 @@ class BaseNode:
         )
         if tag is not None:
             cmd += f" tag {tag}"
+        # Per-link attribution (contract §3.2): when one ubridge bridge serves
+        # several GNS3 links (e.g. IOU's per-node bridge), bridge+filter collide,
+        # so the link id is the only way to tell signals — and pcap files — apart.
+        if link_id:
+            cmd += f" link {link_id}"
         cmd += ' pcap "{path}"'.format(path=pcap_path)
         # Let BPF compile errors propagate — the marker is the user's intent, so a
         # bad expression must surface instead of being silently dropped.
@@ -1133,7 +1138,7 @@ class BaseNode:
                 markers_dir, f"{self._id}_{link_id}_{name}.pcap"
             )
             try:
-                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag)
+                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag, link_id)
             except UbridgeError as e:
                 # Swallow BPF compile errors (warn + skip) so a single bad
                 # expression can't break link creation / node restart — mirrors