feat: cross-compute VXLAN kernel datapath

Cross-compute links can ride kernel VXLAN tunnels instead of the uBridge
UDP relay: each endpoint creates one VXLAN device (uBridge's rtnetlink
vxlan module) enslaved into the same per-link Linux bridge the same-host
datapath uses, so filters, markers, capture and suspend keep running on
the anchor unchanged. Opt-in (Server.enable_vxlan_cross_compute, default
off) because the tunnel caps the usable MTU at the underlay minus the
outer headers; the relay stays the default and the fallback.

The controller derives what the pair must agree on: one VNI per link
(upper VNI space, per-link random salt) and deterministic device names
(gvl{link}{node}) that make crashed-run leftovers claimable
(show -> delete -> create). Same-host compute pairs, switch endpoints
and serial/unix-socket containers stay on the relay.

A VNI the kernel refuses self-heals on every path it can surface:
interactive create retries a fresh salt then degrades to the relay;
project open re-dispatches refused batch entries the same way; node
start recognises the refused device, stops the half-started node,
re-wires both ends on a fresh VNI (or degrades the link) and retries
the start. The device MTU is derived from the lower device (dev=) and
the per-link bridge inherits it - no configuration. The capability
probe (ubridge_vxlan in /capabilities) runs one scratch create/delete
on a scratch port that never equals the configured dstport.

The e2e scenario runs two computes on one machine (a network namespace
joined by a veth pair as the fabric) and needs root.

gns3server/api/routes/compute/capabilities.py

@@ -30,6 +30,7 @@ from gns3server.compute.ubridge.tc_probe import (
     probe_iol_tap_support,
     probe_tap_support,
     probe_tc_capabilities,
+    probe_vxlan_support,
 )
 from gns3server.compute.vpcs.vpcs_probe import probe_vpcs_tap
 from gns3server.utils.path import get_default_project_directory
@@ -75,6 +76,11 @@ async def get_capabilities(request: Request) -> dict:
     # kernel link. None = unknown (probe failed or binary missing).
     vpcs_tap = await probe_vpcs_tap()
 
+    # VXLAN device creation (the vxlan module): the tunnel a cross-compute
+    # kernel link rides. None = unknown (probe failed, uBridge too old, or
+    # the kernel refuses vxlan devices here).
+    ubridge_vxlan = await probe_vxlan_support()
+
     # record the controller hostname or IP address
     if request.client:
         request.app.state.controller_host = request.client.host
@@ -91,4 +97,5 @@ async def get_capabilities(request: Request) -> dict:
         "ubridge_iol_tap": ubridge_iol_tap,
         "ubridge_bridge_tap": ubridge_bridge_tap,
         "vpcs_tap": vpcs_tap,
+        "ubridge_vxlan": ubridge_vxlan,
     }