Importing Red unpacked data/generated, assets/generated and
rom-cache.complete straight into the save-dir root, while Blue and
Yellow land under blue/ and yellow/. Red now uses cachePrefix red/
like the others.
CacheFs.migrateLegacyRedCache moves a pre-existing root cache into
red/ on first boot, from RomImporter.new before the readiness loop
and from mountVersion, so existing installs keep their import instead
of being asked for the ROM again. The move only runs when the root
marker resolves to the save directory, so a dev checkout's source
tree is never touched, and the portable game folder is skipped when
it is the physfs source.
Closes#899
johnjohto