fat_tool.py provides zero-dependency FAT32 manipulation (ls, mkdir,
cp-in, cp-out) using only Python stdlib struct/os. install-grub.sh uses
it to extract the existing Redox bootloader from ESP, install GRUB as
the primary boot manager, and set up the chainload configuration.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
