[xlsx] workbooks created by `create` lack the required `xl/styles.xml` part — Excel opens them but strict OPC parsers fail

Environment

• OfficeCLI v1.0.149 (official release binary, officecli-win-x64.exe, SHA256 verified against SHA256SUMS)
• Windows 11 x64; headless tool→CLI invocation (execFile, direct argv)
• Reproduction does not depend on platform: it is a package-defect of the xlsx writer

Problem

Workbooks created by OfficeCLI lack the xl/styles.xml part entirely. Verified over 5 workbooks from a stress corpus (blank, data-filled, and one with embedded charts): none contain the styles part, [Content_Types].xml has no /xl/styles.xml Override, and xl/_rels/workbook.xml.rels has no workbook→styles relationship.

Excel opens these files fine (the absence is treated as an implied empty stylesheet), but strict OPC parsers fail with entry not found: xl/styles.xml — that is exactly what our integrated viewer reports.

How to verify

node -e "const A=require('adm-zip');const z=new A('hoja.xlsx');console.log(z.getEntries().map(e=>e.entryName).join('\n'))"

Observed entry list for a data-filled workbook:

xl/workbook.xml
xl/worksheets/sheet1.xml
xl/theme/theme1.xml
docProps/core.xml
docProps/app.xml
docProps/custom.xml
_rels/.rels
xl/_rels/workbook.xml.rels
[Content_Types].xml

No xl/styles.xml. [Content_Types].xml has no Override for it; xl/_rels/workbook.xml.rels has no styles relationship. For comparison, docx blanks from the same binary DO include word/styles.xml.

Root cause (verified against source, main @ 1f0fd19)

BlankDocCreator.CreateExcel() adds WorkbookPart + WorksheetPart + ThemePart, but never a WorkbookStylesPart. The styles part only comes into existence when a cell style is applied via ExcelStyleManager.EnsureStylesPart() (set --prop fill/font/numfmt/..., hyperlinks, conditional formatting, table styles, import). A workbook created blank and then filled with plain values or charts stays without the part.

Two observations that support the fix:

1. The docx blank already stamps its StyleDefinitionsPart (BlankDocCreator.CreateWord), and the theme gap for xlsx got the same parity fix this year (theme1.xml — the code comment says "xlsx alone omitted it"); the styles part is the remaining gap of the same kind.
2. The OfficeCLI xlsx reader is fully null-guarded for a missing stylesheet (ExcelHandler.HtmlPreview, ExcelDataFormatter, CheckOverflow/CheckNumericOverflow/CheckGeneralPrecision — all fall back to defaults), so adding the part by default cannot break existing flows.

Suggested fix (minimal)

In BlankDocCreator.CreateExcel(), right after the existing theme part (mirroring the earlier theme fix):

var stylesPart = workbookPart.AddNewPart<DocumentFormat.OpenXml.Packaging.WorkbookStylesPart>();
stylesPart.Stylesheet = ExcelStyleManager.CreateDefaultStylesheet(); // private static → internal static
stylesPart.Stylesheet.Save();

(Reuses the existing minimal stylesheet from ExcelStyleManager. A defensive secondary injection point would be ExcelHandler.Save(), which would also cover files created by import or foreign templates.)

Notes

• We ship a temporary server-side conformance shim (inject the minimal stylesheet + Override + relationship after saving) so our product works meanwhile — but the durable fix belongs in the writer, and we would happily re-test as soon as a release includes it.
• Happy to provide the full 5-file verification corpus if useful.