Part 6 of a stack that makes GRUB installation work without root privileges. Converts the remaining BIOS/MBR path to the same unprivileged approach the EFI path already uses, and deletes the chroot implementation. BIOS booting is fiddlier than EFI because there is no filesystem involved at the start -- the firmware just executes the first sector. So instead of copying files into a partition, we: 1. Build `core.img` on the host with `grub-mkimage`. 2. Copy the boot sector out of GRUB's `boot.img` into sector 0 of the image. 3. Write `core.img` into the post-MBR gap. 4. Patch the boot sector with the sector where `core.img` starts and its blocklist, which is the part `grub-install` would normally do for us. With both firmware types converted, `setup_grub` no longer mounts anything or attaches loop devices at all, so `_grub_install`, `_image_mounts` and the transitional `_setup_grub_bios_chroot` wrapper introduced in the previous PR are removed. Adds `tests/spread/boot/classic-mbr`, which boots a BIOS image under QEMU's SeaBIOS (no OVMF) and asserts it reaches userspace -- i.e. that the hand-written boot sector and blocklist actually work.