PortManager.get_free_udp_port had an unguarded find-then-add sequence.
A link allocates both ends concurrently (asyncio.gather in
UDPLink._prepare -> two POST /ports/udp) and FastAPI runs the sync
route handler in a threadpool, so both threads could probe the same
'free' port before either recorded it — handing lport == rport to both
ends. uBridge sets SO_REUSEADDR on UDP NIO sockets, so the double bind
succeeds silently and the kernel delivers everything to the last-bound
socket: one node starves, the other echoes to itself.
Make every TCP/UDP allocate/reserve/release path atomic with an RLock,
and rebuild _link_data in UDPLink._prepare so reset() commits the fresh
port pair instead of re-sending the stale, already-released one.
Regression tests: threaded barrier allocation never returns duplicates
(red on the old code, UDP and TCP); reset() leaves exactly one mirrored
NIO pair per side with lport != rport (red on the old code).
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.
