feat(ubridge): probe iol_bridge's TAP-port support

Whether an IOU port can terminate on a persistent TAP (iol_bridge
add_nio_tap) is a capability of its own: the tap module and that command
land independently, so a build may have one without the other. Probed on
a scratch IOL bridge with a scratch TAP — one add/delete cycle, no
iol_bridge start needed (attaching a NIO works on a stopped bridge) —
cached by binary identity, with the scratch bridge id above the per-node
id space the fabric locks (513..1024) so a probe cannot collide with a
running node. Any failure, including an old build's "Unknown command",
means unknown: IOU stays on the relay datapath.

Reported through /capabilities as ubridge_iol_tap, next to ubridge_tap.

tests/controller/test_compute.py

@@ -381,7 +381,7 @@ async def test_json(compute):
         "disk_usage_percent": 0,
         "connected": True,
         "last_error": None,
-        "capabilities": {"version": "", "platform": "", "cpus": 0, "memory": 0, "disk_size": 0, "node_types": [], "ubridge_tc": None, "ubridge_tap": None},
+        "capabilities": {"version": "", "platform": "", "cpus": 0, "memory": 0, "disk_size": 0, "node_types": [], "ubridge_tc": None, "ubridge_tap": None, "ubridge_iol_tap": None},
     }
     assert compute.asdict(topology_dump=True) == {
         "compute_id": "my_compute_id",