* Fix Limine install with ESP mounted outside /boot
Place limine.conf next to the EFI binary on the ESP so it is found
regardless of ESP mountpoint, and block unbootable layouts (non-UKI
Limine with ESP not at /boot and no separate /boot partition) in
GlobalMenu validation, guided.main() and _add_limine_bootloader().
Fixes#4333
* Extract bootloader layout validation into lib/bootloader/utils
* Consolidate Limine layout validation in bootloader utils
Move the boot-partition FAT check from GlobalMenu into
validate_bootloader_layout so all three call sites (GlobalMenu,
guided.py, Installer._add_limine_bootloader) share one function.
Return a BootloaderValidationFailure dataclass (kind + description)
instead of str | None, so callers can match on the failure kind and
the description is built where partition context is in scope.
* Encapsulate FAT filesystem detection in FilesystemType.is_fat()
e7d38d0e82