Replaces the hand-rolled subvolume creation / mounting / fstab-append run steps with debos's native btrfs subvolume support in image-partition (declarative subvolumes: + per-mountpoint subvolume:).

Changes (debian-rk3576-img.yaml)

• image-partition now declares the subvolumes (boot, @Minimal, @home, @snapshots, @var-log, @var-cache) and their mountpoints (/, /boot, /home, /var/log, /var/cache) with a per-mount subvolume:; boot gets compression: none so U-Boot can read the kernel/dtb/initrd without a zstd-capable btrfs driver.
• Drops the manual run step that created + nested-mounted the subvolumes, and the one that appended the non-root fstab lines — debos writes them now.
• Adds a soft btrfs balance to assert dup metadata+system / single data (debos passes only mkfs -O/-E, and the default -m/-d has flipped across btrfs-progs versions).
• Drops subvol=@Minimal from the / fstab line so every profile shares a subvol-agnostic fstab and selects its root via rootflags=subvol=.

Blocked on: debos

Requires a debos build with native btrfs subvolume support in image-partition (go-debos/debos#736). Draft until the debos bump lands.