Internal (in-workbook) hyperlinks must be portable across consumers

Cluster: xlsx-io

Scenario

An author creates a hyperlink whose destination is another location inside the same workbook, a cell reference like Sheet2!A1, or a defined name, rather than an external URL. In desktop Excel the link navigates correctly. But when the same file is opened in Google Sheets or WPS Office the link is dead, because those consumers do not resolve it. The cause is how the internal link is serialized: if it is emitted like an external link, with an r:id relationship, instead of with an in-workbook location, stricter consumers silently ignore it.

Spec note, not a corpus case: the correct serialization is knowable, but the observable defect is cross-application navigation, a rendering behavior in third-party apps that the corpus cannot exercise. The durable value is the OOXML shape an internal link must take.

Desired behavior

• An internal hyperlink, to a cell or range on a sheet or to a defined name, is written so non-Excel consumers (Google Sheets, WPS) resolve and navigate to the target, not only Microsoft Excel.
• Concretely: an internal link carries a location attribute holding the in-workbook reference (Sheet2!A1 or a defined name) and does not rely on an external-mode r:id relationship. An external link keeps its r:id pointing at a .rels target with TargetMode="External", and the two forms are serialized distinctly.
• A round-trip preserves the internal target verbatim, whether a sheet-qualified reference or a defined name, and the display text and tooltip survive.

Open questions

• Does the writer infer "internal against external" from the target shape, treating a sheet-qualified reference or a known defined name as internal and a URL scheme as external, or from an explicit caller flag?
• How are internal links to a defined name distinguished from those to a raw cell reference, and are both emitted as location?
• Should an internal target that does not resolve to an existing sheet or name warn on write?

Related: hyperlink-cell-default-style, cross-sheet-reference-preserved-in-formula-and-validation, defined-name-full-row-column-span-survives-roundtrip.