feat(docker): wire Docker-to-Docker links through a kernel bridge

Eligible links (both endpoints Docker, same compute, no filters/markers,
stopped nodes, no unix-socket NIO containers) are wired as veth pairs
enslaved into a per-link Linux kernel bridge (gns3 + 11 hex of the link
id, filling IFNAMSIZ) instead of the uBridge UDP relay, so frames flow
entirely in the kernel.

- NIOBridge NIO type: the controller decides the datapath in
  UDPLink._prepare and both computes enslave their deterministic gv*/gc*
  veth pair via uBridge's docker/brctl modules
- carrier/suspend = host-end admin state; both endpoints race-safely
  create/delete the bridge (EEXIST/EBUSY/ENOENT tolerated)
- veth adapters can still attach relay (UDP) links on a running node:
  lazy relay bridge over AF_PACKET on the host end (add_nio_ethernet)
- filters/markers/capture are rejected with explicit errors on this
  datapath (they live in the uBridge relay); vendor unix-socket
  containers are excluded; Server.enable_kernel_datapath is the
  kill-switch (default on)
- host-end veths survive container death and are removed on stop plus
  swept by name on the next start (crash/stale recovery)

gns3server/compute/base_manager.py

@@ -41,6 +41,7 @@ from ..utils import force_unix_path
 from ..utils.images import default_images_directory, images_directories, list_images, md5sum, remove_checksum
 from .base_node import BaseNode
 from .error import ImageMissingError, NodeError
+from .nios.nio_bridge import NIOBridge
 from .nios.nio_ethernet import NIOEthernet
 from .nios.nio_tap import NIOTAP
 from .nios.nio_udp import NIOUDP
@@ -358,6 +359,11 @@ class BaseManager:
             nio.filters = nio_settings.get("filters", {})
             nio.markers = nio_settings.get("markers", {})
             nio.suspend = nio_settings.get("suspend", False)
+        elif nio_settings["type"] == "nio_bridge":
+            nio = NIOBridge(nio_settings["bridge"])
+            if nio_settings.get("filters") or nio_settings.get("markers"):
+                raise ComputeError("Packet filters and markers are not supported on kernel-datapath links")
+            nio.suspend = nio_settings.get("suspend", False)
         elif nio_settings["type"] == "nio_tap":
             tap_device = nio_settings["tap_device"]
             # if not is_interface_up(tap_device):