marker: serial-link (WAN) support via data_link_type -> uBridge linktype

Markers now work on serial links (Cisco HDLC / PPP / Frame Relay / ATM), not just Ethernet. A marker carries a data_link_type (default DLT_EN10MB); at the uBridge boundary it becomes the 'mark ... linktype <dlt>' keyword so the BPF compiles and the pcap is written with the matching link-layer.

- MarkerCreate / MarkerDefinitionCreate gain data_link_type (default DLT_EN10MB). Per-link it is create-only; definitions are updatable (a change re-fans-out).
- base_node._marker_linktype() normalizes the GNS3 DLT name (strip DLT_, uppercase, None for EN10MB). Single source is SerialPort.data_link_types, so Cisco PPP -> PPP_SERIAL (50), matching the capture path -- no second mapping table.
- _ubridge_add_marker_filter (generic) and the IOU marker loop append 'linktype <dlt>'.
- Definition fan-out branches on link_type in inherit_marker: Ethernet is always EN10MB; a serial link uses the definition's WAN encapsulation, or is SKIPPED when none was chosen (an EN10MB pcap on serial is undecodable). One definition covers a mixed topology.
- MCP marker_definition exposes data_link_type (None = not forwarded).
- No uBridge rebuild on a data_link_type change -- only that one marker's filter is swapped (delete + re-add), mirroring a BPF change; reset_packet_filters preserves sibling mark filters.

Requires the uBridge build with 'mark ... linktype' support.

tests/compute/test_base_node.py

@@ -230,6 +230,52 @@ async def test_apply_markers_turns_disabled_filter_off(compute_project, manager)
     assert node._marker_filter_bridges["m", "L1"] == "VPCS-10"
 
 
+def test_marker_linktype_normalizes():
+    # Ethernet / unset → None (linktype omitted; uBridge defaults to EN10MB).
+    assert VPCSVM._marker_linktype(None) is None
+    assert VPCSVM._marker_linktype("") is None
+    assert VPCSVM._marker_linktype("DLT_EN10MB") is None
+    # Serial DLTs from SerialPort.data_link_types, DLT_ prefix stripped.
+    assert VPCSVM._marker_linktype("DLT_C_HDLC") == "C_HDLC"
+    assert VPCSVM._marker_linktype("DLT_PPP_SERIAL") == "PPP_SERIAL"
+    assert VPCSVM._marker_linktype("DLT_FRELAY") == "FRELAY"
+    assert VPCSVM._marker_linktype("DLT_ATM_RFC1483") == "ATM_RFC1483"
+    # Case-insensitive input → canonical uppercase (pcap_datalink_name_to_val is
+    # case-sensitive and expects the uppercase form). Shared by base_node and IOU.
+    assert VPCSVM._marker_linktype("dlt_c_hdlc") == "C_HDLC"
+
+
+@pytest.mark.asyncio
+async def test_apply_markers_appends_linktype_for_serial(compute_project, manager):
+    # A serial data_link_type reaches the uBridge mark command as `linktype C_HDLC`
+    # so the BPF offsets and pcap decode match the WAN encapsulation.
+    node = VPCSVM("test", "00010203-0405-0607-0809-0a0b0c0d0e0f", compute_project, manager)
+    node._ubridge_send = AsyncioMagicMock()
+    nio = NIOUDP(1234, "127.0.0.1", 4321)
+    nio.markers = {"m": {"bpf": "icmp", "tag": None, "link_id": "L1",
+                         "direction": None, "data_link_type": "DLT_C_HDLC", "enabled": True}}
+    with patch("gns3server.compute.marker.marker_manager.MarkerManager") as mm:
+        mm.instance.return_value.register = MagicMock()
+        await node._ubridge_apply_markers("VPCS-10", nio)
+    sent = [c.args[0] for c in node._ubridge_send.call_args_list]
+    assert any("linktype C_HDLC" in s for s in sent)
+
+
+@pytest.mark.asyncio
+async def test_apply_markers_omits_linktype_for_ethernet(compute_project, manager):
+    # Ethernet (DLT_EN10MB) → no linktype keyword; uBridge defaults to EN10MB.
+    node = VPCSVM("test", "00010203-0405-0607-0809-0a0b0c0d0e0f", compute_project, manager)
+    node._ubridge_send = AsyncioMagicMock()
+    nio = NIOUDP(1234, "127.0.0.1", 4321)
+    nio.markers = {"m": {"bpf": "icmp", "tag": None, "link_id": "L1",
+                         "direction": None, "data_link_type": "DLT_EN10MB", "enabled": True}}
+    with patch("gns3server.compute.marker.marker_manager.MarkerManager") as mm:
+        mm.instance.return_value.register = MagicMock()
+        await node._ubridge_apply_markers("VPCS-10", nio)
+    sent = [c.args[0] for c in node._ubridge_send.call_args_list]
+    assert not any("linktype" in s for s in sent)
+
+
 @pytest.mark.asyncio
 async def test_delete_marker_capture_removes_pcap_and_entry(compute_project, manager):
     # Deleting a marker's capture removes its pcap and forgets the bridge entry.