README: add a bread-ecosystem feature matrix, keyboard-shortcut reference,
a Known Limitations section (NVIDIA/Mesa, VM GPU accel, Secure Boot, btrfs
assumption), and a Recovery guide (snapshot rollback + GRUB/EFI repair from
the live ISO).
scripts/smoke-test.sh: read-only post-install validator — btrfs subvolumes,
snapper config, enabled services, bread bins on PATH, bos-settings, default
dotfiles, and the GRUB EFI artifacts. Exits non-zero on any failure.
