feat(controller): expose kernel_datapath on the link payload

Runtime fact recomputed from the NIO wiring on every load (not
persisted). Without it clients cannot tell a relay link from a kernel
link whose compute lacks a filter capability — both show up as
kernel-only types missing from available_filters.

tests/controller/test_link.py

@@ -228,6 +228,7 @@ async def test_json(project, compute):
         "suspend": False,
         "wireshark": False,
         "link_type": "ethernet",
+        "kernel_datapath": False,
         "capturing": False,
         "capture_file_name": None,
         "capture_file_path": None,