docs: record the link UDP self-loop bug; mark XRd datapath validated

Add bugs/link-udp-self-loop.md: the intermittent one-way docker link
observed during XRd validation (one end's nio_udp rport pointing at its
own lport after a uBridge restart), with the evidence table, the uBridge
console capture diagnostics, the delete/recreate workaround, and the
narrowed suspects (batch port preallocation / link re-creation race).
Also note that a docker node's in-container ethN is a TAP device held by
uBridge (no veth host end exists).

Update the XRd feature doc: datapath validated end-to-end (XRd brings
its own interfaces up, ARP/ICMP bidirectional), add a troubleshooting
row pointing at the bug doc. Index the bug in docs/README.md.

docs/features/vendor-nos-xrd.md

@@ -158,6 +158,7 @@ sequenceDiagram
 | `Invalid interface entries ... XR_MGMT_INTERFACES` | use `xr_name=Mg0/RP0/CPU0/0` |
 | Host audio muted / USB reconnects when the node starts | set `GNS3_MASK_UDEV=1` |
 | Config lost across stop/start | `extra_volumes` must be `/xr-storage-shadow` |
+| Two nodes can't ping, only one side ARPs | GNS3 link wiring bug (UDP self-loop on one end), not XRd — delete and re-create the link; see [link-udp-self-loop](../bugs/link-udp-self-loop.md) |
 | Compute log: busybox coredump storm | fixed by the container-chown change; verify gns3-server is current |
 
 ## Notes
@@ -190,4 +191,5 @@ sequenceDiagram
 
 | Version | Date | Changes |
 |---------|------|---------|
+| 1.1 | 2026-08-14 | Datapath validated end-to-end (XRd brings its own interfaces up; ARP/ICMP bidirectional). Add troubleshooting entry for the one-way-link symptom (GNS3 link UDP self-loop bug, see bugs/link-udp-self-loop.md). |
 | 1.0 | 2026-08-14 | Initial documentation of the XRd control-plane adaptation: vendor path requirement, shm/devices/extra_configs/udev-mask mechanisms, host-disturbance root causes, appliance recipe. |