feat: threaded comments — Excel 365+ modern comment system

[productdevbook]

Summary

Support threaded comments (Excel 365+), distinct from legacy comments/notes.

Technical details

Stored in xl/threadedComments/threadedComment1.xml:

• Thread-based replies via parentId
• Author info: userId, providerId, displayName
• Timestamps on each comment
• Excel auto-creates legacy comment placeholders for backward compat

Scope

• Read threaded comments
• Write threaded comments
• Reply threading (parentId)
• Author + timestamp metadata
• Auto-generate legacy comment placeholders
• Round-trip preservation

References

• MS-XLSX: https://learn.microsoft.com/en-us/openspecs/office_standards/ms-xlsx/e0fb917a-1107-409a-852f-13b47aea70dc