Add /boot root mount planning for VFAT ESP outputs, reuse the idempotent
mount logic to skip duplicates, and ensure /etc/fstab includes both
/var/mounts/{UUID} and the ESP when enabled.
* read /proc/self/mountinfo to detect already mounted targets and reuse their metadata
* reject conflicting mounts by source, allowing reruns without duplicating entries
* return mount results sorted by target for deterministic downstream behavior
* write subvolume and /var/mounts/{UUID} entries to fstab when requested