Graduate the proven Spike 3 storage stack into firmware::persistence so the
editor and the spike share one implementation. Storage::mount brings up the
dedicated SPI3 bus (ADR-012) and mounts FAT with format_if_mount_failed=false
(never reformat the user's card). save() does the FAT-safe atomic write
(tmp -> fsync -> unlink -> rename); recover() reconciles a leftover *.tmp at
boot, keeping the committed file when the crash point is ambiguous and only
promoting the tmp when the target was already unlinked. load() caps reads at
256 KiB.
