Documents the two-phase GRUB integration approach (post-build script
then installer-native), build notes for the three cookbook workarounds,
and measured artifact sizes. Sets efi_partition_size=16 in redbear-full
to accommodate GRUB plus Redox bootloader on the ESP.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
