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).
