Two phases are now announced from the command that serves traffic, so a module
can attach to them instead of being called by name from here.
The listener is opened by this goroutine rather than left to ListenAndServe,
which binds on the goroutine that serves. That mattered for the phase: a hook
on AfterListen is promised a reachable port, and with the bind happening out of
sight there was no way to keep that promise - "address already in use" surfaced
on a goroutine nobody read, after the banner had already announced the server
was up. It is now returned from run() and the process exits non-zero without
claiming anything.
AfterListen is announced synchronously. Moving it to a goroutine to save the
few milliseconds would let it overlap the shutdown, and on a fast SIGTERM the
cleanup callbacks could finish before the startup ones did.
What is left in the serving goroutine is still log.Fatal, deliberately: the
bind is no longer among the errors that reach it, so what remains is a serve
that failed after the port was taken, and carrying on would park the process on
<-quit with nothing serving. ServeTLS is the one case that can still fail
immediately, since it reads the certificate files - with ssl enabled a hook can
still run against a server on its way down. That is not a regression (the old
code printed the banner in the same situation) and it is not fixed here.
BeforeRouter is placed before initRouter, which is a different moment from the
before registry runStartupHooks drains: those callbacks run after the engine
has been built, not before it.
AfterListen is tested here, in one test rather than two because the phase seals
itself once it has run: a second test would find a closed registry and pass
while proving nothing. Both counter-proofs compile and fail - announcing on a
failed bind reports "AfterListen ran 1 times after a failed bind", and
`go RunPhase(...)` reports "ran 0 times, want 1" against the hook's own pause.
BeforeRouter's placement is not asserted in this commit. The test for it comes
with the buildRouter extraction later in this branch.
