gnome-shell keeps segfaulting in libmutter, and with the PAM core
limit at 0 systemd-coredump threw every dump away, so there was no
backtrace to work from. Raise the limit to unlimited; dumps land
root-owned in /var/lib/systemd/coredump, readable only by their
owner. Enable nixseparatedebuginfod2 so gdb/coredumpctl can fetch
symbols for store paths on demand.
The user manager inherits the limit at its first login, so it takes
a reboot (or a full logout) to apply.
Assisted-by: Claude:opus-5
