A read→write round-trip must preserve row/column visibility

The scenario

A user opens an existing .xlsx, makes no edits or a single-cell edit, and writes it back. The output refuses to open in Excel and LibreOffice, which reject it, while a lenient in-browser viewer still renders it. The user traced the difference to every row and column being written as hidden, and manually re-setting visibility made the file open again. A no-op round-trip must never turn a visible sheet into a wholly hidden one, and must never emit a document mainstream consumers reject.

Desired behaviour

• Row and column visibility (hidden) round-trips faithfully: a row or column visible on read is visible on write, and a genuinely hidden one stays hidden. No path defaults visibility to hidden.
• A sheet that was not wholly hidden on input is never serialized as wholly hidden.
• The written package is accepted by all mainstream consumers (Excel, LibreOffice, Sheets), not only lenient viewers, so this is also a structural-validity concern rather than only a visibility-flag concern (see [[excel-repair-on-open-structural-constraints]]).

Root cause (legacy)

Unconfirmed. The reported symptom is "all rows and columns hidden after a no-op round-trip," but it is not yet isolated whether the true defect is a visibility flag mis-serialized on the read-then-write path, say a default hidden leaking onto every row and column model, or a separate structural corruption that merely manifested as an all-hidden sheet in the viewer that could open it. The two have different fixes.

Open questions for the rebuild

• Is the defect visibility mis-serialization, or a structural package corruption that presents as "all hidden"? Disambiguating needs a reproducing input file, and the original was only ever shared via a since-dead link, so this is recorded as design intent rather than a corpus lock.
• Where does a default hidden value come from on the row and column model, and can the model distinguish "explicitly visible" from "unset" so a round-trip never invents a hidden flag?
• Should the writer refuse to emit a wholly-hidden workbook, since Excel treats a workbook with no visible sheet as invalid, or warn? This overlaps the write-time validation policy in [[excel-repair-on-open-structural-constraints]].