A new constructor takes a numeric bind address (127.0.0.1, ::1, 0.0.0.0,
::) and accepts port 0, and Port() reports the port the listener got.
Until now every listener bound 0.0.0.0 at a port the caller chose, so a
helper meant for this machine's own processes was reachable from the
network, and the caller had to guess a free port. A name such as
`localhost` is refused instead of bound to whichever address the
resolver ranks first. The port-only constructor still listens on every
IPv4 interface.
`running` is atomic now: Stop() runs on another thread than the accept
loop that reads it. The accept loops take a sockaddr_storage, so an IPv6
peer's address is no longer cut short.
New: ShouldBindLoopbackEphemeral.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
