feat: implement traffic insight marker backend (ubridge mark filter)

Add compute-side marker subsystem that receives ubridge UDP MARK signals
and forwards them as project-scoped notifications to the web UI for
real-time traffic coloring. Matched packets are always saved to per-link
pcaps for future replay.

Key components:
- gns3server/compute/marker/: MarkerManager (singleton, UDP listener +
  O(1) registry keyed by (node_id, filter_name)) + MarkerListener
  (DatagramProtocol parsing MARK lines per ubridge integration contract)
- gns3server/compute/base_node.py: marker sink/node config at ubridge
  startup; shared _ubridge_add_marker_filter / _ubridge_delete_marker_filter
- Per-node start_marker/stop_marker: VPCS (VPCS-{id}), QEMU
  (QEMU-{id}-{adapter}), Docker (bridge{adapter})
- Compute REST /markers/start + /markers/stop (vpcs/qemu/docker route files)
- Controller Link._markers state + UDPLink.start_marker/stop_marker/
  update_marker (mirror capture pattern: BPF validation, _choose_capture_side,
  node.post forwarding, topology persistence)
- Controller REST GET/POST/DELETE/PUT /v3/projects/{p}/links/{l}/markers
- Config: marker_listen_host / marker_listen_port in ServerSettings
- Signal routing: creation-time registry O(1) lookup, no node-table scan;
  project-scoped WS stream (not global); event payload always carries
  project_id for frontend scoping

Tests: 14 unit tests (registry, listener parsing, UDP round-trip);
562 existing tests pass with zero regressions.

tests/compute/marker/test_marker_manager.py

@@ -0,0 +1,218 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 GNS3 Technologies Inc.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import asyncio
+import pytest
+
+from gns3server.compute.marker.marker_manager import MarkerManager
+from gns3server.compute.marker.marker_listener import MarkerListener
+
+
+# ---------------------------------------------------------------------------
+# Registry
+# ---------------------------------------------------------------------------
+
+class TestMarkerRegistry:
+
+    def test_register_and_lookup(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+        mgr.register("proj1", "node1", "filter1", "link1", tag=5)
+        pid, lid, tag = mgr.lookup("node1", "filter1")
+        assert pid == "proj1"
+        assert lid == "link1"
+        assert tag == 5
+
+    def test_miss_returns_none(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+        pid, lid, tag = mgr.lookup("no-such-node", "no-such-filter")
+        assert pid is None
+
+    def test_reregister_updates(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+        mgr.register("p", "n", "f", "l", tag=1)
+        mgr.register("p", "n", "f", "l", tag=99)
+        _, _, tag = mgr.lookup("n", "f")
+        assert tag == 99
+
+    def test_unregister(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+        mgr.register("p", "n", "f", "l")
+        assert mgr.unregister("n", "f") is True
+        pid, _, _ = mgr.lookup("n", "f")
+        assert pid is None
+        assert mgr.unregister("n", "f") is False
+
+    def test_unregister_project(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+        mgr.register("p1", "n1", "f1", "l1")
+        mgr.register("p1", "n2", "f2", "l2")
+        mgr.register("p2", "n3", "f3", "l3")
+        mgr.unregister_project("p1")
+        assert mgr.lookup("n1", "f1") == (None, None, None)
+        assert mgr.lookup("n2", "f2") == (None, None, None)
+        assert mgr.lookup("n3", "f3")[0] == "p2"
+
+    def test_re_add_after_project_clear(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+        mgr.register("p", "n", "f", "l")
+        mgr.unregister_project("p")
+        mgr.register("p", "n", "f", "l2", tag=42)
+        pid, lid, tag = mgr.lookup("n", "f")
+        assert pid == "p" and lid == "l2" and tag == 42
+
+
+# ---------------------------------------------------------------------------
+# MarkerListener parsing
+# ---------------------------------------------------------------------------
+
+class FakeMarkerManager:
+    def __init__(self):
+        self.events = []
+        self._entries = {}
+
+    def lookup(self, node_id, filter_name):
+        e = self._entries.get((node_id, filter_name))
+        if e is None:
+            return None, None, None
+        return e["project_id"], e["link_id"], e["tag"]
+
+    def emit_match(self, project_id, event):
+        self.events.append((project_id, event))
+
+    def register(self, project_id, node_id, filter_name, link_id, tag):
+        self._entries[(node_id, filter_name)] = {
+            "project_id": project_id, "link_id": link_id, "tag": tag
+        }
+
+
+class TestMarkerListener:
+
+    def test_parses_valid_mark_datagram(self):
+        fmgr = FakeMarkerManager()
+        fmgr.register("p1", "n1", "f1", "l1", tag=7)
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(
+            b"MARK 1700000000.123456 node=n1 filter=f1 tag=7 len=98\n",
+            ("127.0.0.1", 9999),
+        )
+        assert len(fmgr.events) == 1
+        _, ev = fmgr.events[0]
+        assert ev["node_id"] == "n1"
+        assert ev["link_id"] == "l1"
+        assert ev["filter"] == "f1"
+        assert ev["tag"] == "7"
+        assert ev["ts"] == pytest.approx(1700000000.123456)
+        assert ev["len"] == 98
+
+    def test_unknown_node_dropped(self):
+        fmgr = FakeMarkerManager()
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK 1.0 node=bad filter=bad len=10\n", None)
+        assert fmgr.events == []
+
+    def test_bad_timestamp_ignored(self):
+        fmgr = FakeMarkerManager()
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK badts node=n filter=f len=1\n", None)
+        assert fmgr.events == []
+
+    def test_not_mark_line_ignored(self):
+        fmgr = FakeMarkerManager()
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"HELLO world\n", None)
+        assert fmgr.events == []
+
+    def test_missing_node_ignored(self):
+        fmgr = FakeMarkerManager()
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK 1.0 filter=f len=1\n", None)
+        assert fmgr.events == []
+
+    def test_tag_dash_falls_back_to_registered(self):
+        fmgr = FakeMarkerManager()
+        fmgr.register("p", "n", "f", "l", tag=42)
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK 2.0 node=n filter=f tag=- len=20\n", None)
+        assert fmgr.events[0][1]["tag"] == 42
+
+    def test_exception_does_not_kill_listener(self):
+        fmgr = FakeMarkerManager()
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        # Non-decodable bytes
+        lis.datagram_received(b"\xff\xfe\xfd", None)
+        # The listener swallows exceptions; reaching here proves it survived.
+        assert True
+
+
+# ---------------------------------------------------------------------------
+# UDP round-trip
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+class TestMarkerManagerUDP:
+
+    async def test_listener_receives_and_dispatches(self):
+        MarkerManager.reset()
+        mgr = MarkerManager.instance()
+
+        captured = []
+        original_emit = mgr.emit_match
+        mgr.emit_match = lambda pid, ev: captured.append((pid, ev))
+
+        await mgr.start("127.0.0.1", 0)
+        assert mgr.running
+        assert mgr.port is not None
+
+        mgr.register("proj-rt", "node-rt", "filt-rt", "link-rt", tag=10)
+
+        loop = asyncio.get_running_loop()
+
+        class SendProto(asyncio.DatagramProtocol):
+            def connection_made(self, transport):
+                self.transport = transport
+
+        sp = SendProto()
+        transport, _ = await loop.create_datagram_endpoint(
+            lambda: sp, remote_addr=("127.0.0.1", mgr.port)
+        )
+        transport.sendto(
+            b"MARK 123.456 node=node-rt filter=filt-rt tag=10 len=88\n"
+        )
+        await asyncio.sleep(0.15)
+        transport.close()
+
+        mgr.emit_match = original_emit
+        await mgr.stop()
+
+        assert len(captured) == 1
+        pid, ev = captured[0]
+        assert pid == "proj-rt"
+        assert ev["link_id"] == "link-rt"
+        assert ev["len"] == 88