perf: batch NIO dispatch on project open (one HTTP per compute)

Project open used to create each link by issuing two NIO POSTs from the
controller to the compute — ~5000 HTTP round-trips for a 2500-link
topology, all funnelling through the single shared controller/compute
event loop and capping throughput near 12 links/s.

Replace it with a bulk path:
- UDPLink split into _prepare() (local: ports, peer addrs, link_data)
  and _commit_nios() (dispatch). create() = prepare + commit (interactive).
- Link.add_node gains batch=True: attach both nodes without triggering
  per-link NIO HTTP.
- compute: new POST /projects/{id}/nios/batch endpoint with a unified
  _add_nio_binding dispatch across node types (docker/qemu/iou/vpcs/
  builtin differ in signature).
- project.open: prepare all links locally, group NIO entries by compute,
  send each compute a single /nios/batch, then finalise (wire node/port
  refs, mark created, notify, apply marker defs) in parallel.

Cuts controller->compute HTTP from O(links) to O(computes). Test added
for the batch endpoint.

tests/api/routes/compute/test_docker_nodes.py

@@ -211,6 +211,34 @@ class TestDockerNodesRoutes:
         assert response.status_code == status.HTTP_201_CREATED
         assert response.json()["type"] == "nio_udp"
 
+    async def test_docker_nio_batch_create(self, app: FastAPI, compute_client: AsyncClient, vm: dict) -> None:
+        """
+        Exercise the project-wide batch NIO endpoint: bind two NIOs on the same
+        docker node in a single request (the path used during project open).
+        """
+
+        params = {
+            "nios": [
+                {
+                    "node_id": vm["node_id"],
+                    "adapter_number": 0,
+                    "port_number": 0,
+                    "nio": {"type": "nio_udp", "lport": 4242, "rport": 4343, "rhost": "127.0.0.1"},
+                },
+                {
+                    "node_id": vm["node_id"],
+                    "adapter_number": 1,
+                    "port_number": 0,
+                    "nio": {"type": "nio_udp", "lport": 4243, "rport": 4344, "rhost": "127.0.0.1"},
+                },
+            ]
+        }
+
+        url = app.url_path_for("compute:create_batch_nios", project_id=vm["project_id"])
+        response = await compute_client.post(url, json=params)
+        assert response.status_code == status.HTTP_201_CREATED
+        assert response.json()["added"] == 2
+
 
     async def test_docker_update_nio(self, app: FastAPI, compute_client: AsyncClient, vm: dict) -> None: