Leave a grub.cfg on BIOS installs so GRUB-payload firmware boots

[felixzsh]

What

On BIOS installs, also leave a minimal grub.cfg on the ESP so firmware that
boots through a GRUB payload can hand off to the installed system.

Why

Some coreboot-based firmware boots via a GRUB payload that scans disks for a
boot config (grub.cfg / syslinux / extlinux) and does not chainload the MBR.
Omarchy installs Limine, which writes limine.conf; that firmware does not read
it, so the install completes but the machine does not start — the payload drops
into its scanner with .../i386-coreboot/*.mod not found noise.

The MBR already receives Limine (limine bios-install), but the payload never
boots the MBR on its own: it only looks for a config file. Leaving it one is the
missing piece.

How

In finalize_limine_boot, after Limine is finalized, non-UEFI installs now get a
grub/grub.cfg on the ESP next to limine.conf:

set timeout=3
menuentry "Omarchy" {
  linux /<machine-id>/<kernel>/vmlinuz <cmdline>
  initrd /<ucode> /<machine-id>/<kernel>/initramfs
}

The kernel paths are read back from the generated limine.conf
(path: / module_path:, stripping the boot(): prefix and the content hash),
and the cmdline is the same one Limine uses, so cryptdevice / rootflags /
resume stay correct. intel-ucode.img / amd-ucode.img is prepended when
present.

Impact on normal installs

None.

• UEFI installs: the code does not run.
• Legacy BIOS installs: the file is written but never read. The firmware boots
  the MBR Limine, and Limine reads limine.conf; the file is inert.
• It does not touch the MBR, limine.conf, snapshots, or kernel updates.

Testing

• Added test/unit/test_bios_grub_config.py: path parsing, the generated config,
  and the no-op case when the entry has no kernel/initramfs paths.
• ./test/all passes (72 unit tests).
• The generated config was validated by hand on a no-EFI coreboot machine: with
  the file present the GRUB payload boots the installed system; without it, it
  does not.

Notes

• Limine remains the real bootloader and limine-snapper-sync keeps managing
  limine.conf. Snapshot entries are not mirrored into this grub.cfg; booting
  a snapshot still goes through Limine. This file is only the handoff for
  firmware that cannot read limine.conf.
• A follow-up could chainload the MBR Limine from this file instead, but the
  direct kernel entry works regardless of what is (or is not) in the MBR.
• Automates the manual live-USB workaround people have been using for this class
  of firmware (discussion Boot fails on first boot with Libreboot/Coreboot (no EFI payload) — installer should detect non‑EFI and offer legacy/BIOS install omarchy#4209 ).