Closes#43
FBNeo and Kronos expect BIOS archives in core-specific subdirectories
(system/fbneo/, system/kronos/). RetroArch firmware check uses .info
paths which include these prefixes, so files at root show as Missing.
Add archive_prefix field to emulator profiles. The pack code now places
archive copies in the prefixed subdirectory while keeping root copies
for cores that expect them there (e.g. Geolith for neogeo.zip).