SegmentHeader

A segment's header is unwritten, torn, fails its checksum — or the segment could not be read at all.

"Could not be read" belongs here rather than in a constant of its own because the remedy is identical: a file a backend cannot open (an Apple device still locked under Data Protection, a descriptor limit, an interrupted read) may open perfectly on the next attempt, exactly as a torn header may be completed by a writer that catches up. Where a backend has an errno to report, it reports it out of band rather than by splitting this constant.