The loader ENUMERATES /system now, so it sees whatever the stick's
directory entries literally store — and a hand-copied stick differs
from our generated image: firmware returns bare 8.3 short entries
UPPERCASE (INIT, SYSTEM), and host OSes leave litter next to every file
(macOS '._' AppleDouble forks, .fseventsd). Verified in QEMU/OVMF: an
uppercase-stored volume booted to a dead kernel-only system before this
change and boots fully (display up, logger writing /var/log) after.
The walk now lowers ASCII names (the danos tree is canonically
lowercase; FAT lookups are case-insensitive by definition), skips any
dot-prefixed entry, and treats malformed or unreadable entries as
skip-this-file instead of abort-the-whole-walk. Reader.find compares
case-insensitively as belt and braces.
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