Interface JSON

mc:Ignorable="x14 xxl21" was present on the part root — preserved so a future writer round-trips it.

One entry per CACHED sheet, not per sheet — a sheet absent here was never cached, and reads blank.

Absent between the link visitor and XLSXReader: a visitor never sees its own part's sibling .rels file, so the reader attaches these afterwards. getSources() reads it as empty.