feat(controller): offer QEMU the kernel datapath, gated per compute

Kernel eligibility becomes "both endpoints on one compute, each able to
anchor": Docker always (adapters are born as veth pairs), QEMU when that
compute's uBridge can create the persistent TAPs its adapters anchor on. The
question is asked per compute, because the link needs both ends anchored --
so a mixed or half-upgraded deployment degrades to the relay instead of
failing an attach.

probe_tap_support() creates and deletes one throwaway persistent TAP through a
scratch uBridge, cached by binary identity like the tc report; /capabilities
exposes `ubridge_tap` (absent = unknown, which keeps links on the relay).

The compute route for a QEMU adapter NIO took UDPNIO only, so a kernel link
was rejected with 422: its body and response now accept
Union[UDPNIO, BridgeNIO] exactly like Docker's.

Stale "Docker-to-Docker" wording in the config, link and MCP descriptions now
talks about anchors, and docs/features/qemu-kernel-datapath.md documents the
datapath -- verified end-to-end on a real instance: 32/32 kernel-datapath
checks and 29/29 on the relay variant.

tests/api/routes/compute/test_capabilities.py

@@ -28,6 +28,12 @@ from gns3server.version import __version__
 pytestmark = pytest.mark.asyncio
 
 
+async def _no_tap_probe():
+    """The tap probe spawns uBridge — stub it out for payload assertions."""
+
+    return None
+
+
 class TestCapabilitiesRoutes:
     async def test_get(self, app: FastAPI, compute_client: AsyncClient, windows_platform, monkeypatch) -> None:
 
@@ -35,6 +41,7 @@ class TestCapabilitiesRoutes:
             return None
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tc_capabilities", no_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json() == {
@@ -59,6 +66,7 @@ class TestCapabilitiesRoutes:
             "memory": psutil.virtual_memory().total,
             "disk_size": psutil.disk_usage(get_default_project_directory()).total,
             "ubridge_tc": None,
+            "ubridge_tap": None,
         }
 
     async def test_get_on_gns3vm(self, app: FastAPI, compute_client: AsyncClient, on_gns3vm, monkeypatch) -> None:
@@ -67,6 +75,7 @@ class TestCapabilitiesRoutes:
             return None
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tc_capabilities", no_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json() == {
@@ -91,6 +100,7 @@ class TestCapabilitiesRoutes:
             "memory": psutil.virtual_memory().total,
             "disk_size": psutil.disk_usage(get_default_project_directory()).total,
             "ubridge_tc": None,
+            "ubridge_tap": None,
         }
 
     async def test_get_reports_ubridge_tc(self, app: FastAPI, compute_client: AsyncClient, monkeypatch) -> None:
@@ -99,6 +109,7 @@ class TestCapabilitiesRoutes:
             return {"netem": "delay,rate", "ebpf": "1", "cbpf": "0", "ebpf_modes": "nth,quota,window,flow"}
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tc_capabilities", fake_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json()["ubridge_tc"] == {
@@ -107,3 +118,17 @@ class TestCapabilitiesRoutes:
             "ebpf_modes": ["nth", "quota", "window", "flow"],
             "cbpf": False,
         }
+
+    async def test_get_reports_ubridge_tap(self, app: FastAPI, compute_client: AsyncClient, monkeypatch) -> None:
+        """
+        The tap capability is what makes a QEMU node kernel-eligible, so it
+        travels with the other compute capabilities.
+        """
+
+        async def fake_tap_probe():
+            return True
+
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", fake_tap_probe)
+        response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
+        assert response.status_code == status.HTTP_200_OK
+        assert response.json()["ubridge_tap"] is True