--all --system built every pack asked for and exited 1 on a platform
that has no such system. The scope is now decided before the lock:
skipped under --all, refused under --platform (finding 15).
A system the target leaves no core for was verified 0/0 with exit 0,
and generate_pack blamed the platform for lacking it. Both read
platform_scope_systems and system_scope_error now (findings 13, 14).
A platform without hardware targets is now left out of a targeted --all run instead of being built unfiltered, and a run no platform can honour is refused.