`isSourceRunning` treated `.parentlock` / `parent.lock` as proof Firefox held
the profile, but Firefox deliberately leaves those on disk after a clean exit
(as a last-used marker) and only releases the OS-level fcntl or handle lock —
so once Firefox had been used, the source reported `browserRunning` forever and
cookie import never proceeded.

Verified against Firefox's nsProfileLock.cpp: only the Linux `lock` symlink
(target `<ip>:[+]<pid>`) is unlinked on exit, so its presence plus a live pid
is the POSIX signal; a dead pid means a crash. Node has no fcntl probe, so
`.parentlock` carries no signal on POSIX. On Windows the held handle denies our
open as `Busy`, which is the signal there.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>