Development record

fsutil.CaseFoldingFS decides whether names fold by the operating system (darwin or windows), not by the volume a path is on. A case-sensitive APFS volume on macOS only over-matches, the safe side; a case-insensitive mount on Linux (SMB, exFAT) under-matches, so every check that folds through it (the loop's checked-out check, the FoldPath callers) reads two spellings of one name there as distinct.