Agent-view showed "—" device + no size for a raw (no-PVE-storage) drive because
the registry union row never set backing_device/total_bytes/used_bytes (Observe
drives get those from pvesm status). Resolve BackingDevice via ByUUIDDevicePath +
read capacity via statfsCapacity (build-tagged syscall.Statfs; no-op off-Linux).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
