The reader must not crash on a foreign-shaped rich-text shared string

Cluster: types

Scenario

A workbook contains a cell whose value is rich text, multiple styled runs, stored in the shared strings table, including content that represents a hyperlink expressed as rich text within a shared string. When the reader parses that shared string it must accumulate the <r> runs into a rich-text value object. A brittle parser assumes its accumulator is already an object, but for certain foreign-generated shapes it is handed an empty string primitive instead, so setting the run collection onto it throws Cannot create property 'richText' on string ''. The result is that an otherwise valid workbook fails to open at all.

Spec note, not a corpus case: probing shows the library reads its own rich-text shared strings, including runs with fonts, without throwing, and the concatenated text and runs surface correctly. So the crash depends on a specific foreign-generator shape, an empty-string accumulator seeded before the runs, such as a leading empty <t/> or a hyperlink-as-rich-text quirk, that is not reproducible from the library's own output. The durable value is the robustness requirement and the exact failure signature; promote to a fixture-backed corpus case once a reliably-reproducing foreign sample is captured, the same posture as rich-text-cell-value-writer-robustness on the write side.

Desired behavior

• Reading a shared string with rich-text runs never throws, regardless of how the <si> was shaped by a foreign generator, whether the entry leads with an empty text node, mixes a plain <t> with <r> runs, or carries a hyperlink expressed as rich text. The run accumulator must be initialized as the correct container up front, not assumed to already be one, and never value.richText = … on a value that may still be a primitive string.
• The cell surfaces its content after read. The affected cell reads back as a rich-text value whose concatenated run text equals the intended string, so downstream consumers see the text rather than a load failure.
• Foreign-generator tolerance is the frame. This is one instance of the broader rule that the reader must tolerate structurally-valid-but-unusual OOXML from non-Excel producers without an unguarded property access bringing down the whole open.

Open questions

• What are the concrete foreign shapes that seed an empty-string accumulator? Capture at least one as a fixture (hyperlink-as-rich-text, or a leading empty <t/> before runs) to lock the regression.
• Should a malformed run, missing <t> or empty, contribute an empty string to the concatenation, or be skipped? Define it so the read is deterministic.
• Does the same accumulator-initialization bug affect inline strings (<is> on the cell) as well as shared strings? Audit both paths.

Related: rich-text-cell-value-writer-robustness, streaming-read-resolves-shared-strings, hyperlink-display-text-can-be-rich-text, html-fragment-to-rich-text-cell-value.