perf(convert): reduce rootfs copies from 3 to 1 using qcow2 overlay snapshots

Replace the intermediate rootfs.img file and final qemu-img convert step
with qcow2 overlay/snapshot architecture. The source image is modified
through a writable overlay (source-mod), then forked into two independent
snapshots (source-read for verity hash, source-write for dracut). This
eliminates two full rootfs copies (~17s dd + ~32s qemu-img convert).

Additional fixes:
- Fix RPM version detection: rpm -q outputs error to stdout on uninstalled
  packages, which was captured as the version string
- Fix unbound boot_part variable when source has no separate boot partition

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: imlk0