…ility

Dynamically generated sheetData content was missing namespace prefixes,
causing XML inconsistency when worksheet root uses 'x:' prefix.

Symptoms: Excel opens the file with correct file size but displays an
empty worksheet - data is silently ignored due to malformed XML.

When saving, inconsistent namespace prefixes were used, resulting in
invalid XML documents.  The prefix is now dynamically determined from the
document and used consistently across all elements.

- Extract _nsPrefix from WorksheetXml. DocumentElement
- Replace all hardcoded "x:" prefixes with _nsPrefix
- Add using alias RegexMatch to resolve naming conflict
- Fix $"" combined with AppendFormat/string.Format calls

Co-authored-by: Copilot Autofix powered by AI <[email protected]>

fix: Add x: namespace prefix to sheetData elements for Excel compatib…