Keep the TUIC stream fragmentation check on a working UDP loopback path,
allocate the AmneziaWG fixture ports dynamically, and wait for a real
Xray stats response before starting API mutation assertions. Recreate
failed readiness channels so an early connection cannot stall the probe.
Verified host and offline parallel Go suites, repeated occupied-port and
UDP tests, race/shuffle runs, and golangci-lint. Production behavior and
dependency versions are unchanged.
Split out of #6877 so that pull request's squash carries only the feature.