Builds GRUB 2.12 for the host machine (not Redox target) using template=custom.
Produces a 540 KiB standalone PE32+ EFI binary with GPT, FAT, ext2,
chainloader, and utility modules. The grub.cfg chainloads the Redox
bootloader from EFI/REDBEAR/redbear.efi.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
