The destination name has one-second granularity and mv overwrites.
Closing, re-hosting and closing the same party name within a second
would lose the first transcript while reporting success. Fall back to
a PID-suffixed name when the destination already exists.
A server crash mid-logged-party is the one case close's rescue can
never have run, and host's EEXIST message funnels the host straight
into clean. Deleting the night's log there is exactly the accident
class the close-time rescue exists for, so clean applies the same
rescue: no quiescence poll needed, the liveness gate just proved the
writers are gone.
Turns the manual rm -rf recovery from host's EEXIST message into a
verb, behind the same gates cmd_close uses: own-basename glob, no
symlinks, ownership check, and a dead socket. Live parties are skipped
with a pointer to party close.
