Fix rich text rendering in PDF export and stop mutating source workbook

[swmal]

Summary

Fixes a rich text rendering bug in PDF export and removes a serious side effect where the export mutated the source workbook.

Changes

• Rich text fix: PdfTextMap now reads cell.RichText via the public property (which lazy-loads from XML) instead of the internal _rtc field. Cells whose _rtc happened to be null previously had their rich text formatting flattened to a single run of plain text.
• No workbook mutation: GetTextFragments no longer creates an ExcelRichTextCollection for plain-text cells. That constructor had a hidden side effect — it set CellFlags.RichText and overwrote cell.Value, silently corrupting user data on export.
• Stream API: Added PdfCatalog(settings, worksheet, stream) and ExcelPdf.CreatePdf(stream) overloads for in-memory export.
• Temp worksheet cleanup: The temporary worksheet used for comments/notes pages is now removed after export. Previously it remained in the workbook, causing subsequent exports of the same workbook to fail.
• ExcelColor.ToColor(): New public helper that resolves theme, tint, indexed, and RGB to a System.Drawing.Color.

How to verify

Export a workbook containing both rich-text and plain-text cells, then export it again from the same ExcelPackage instance. Both exports should succeed and render rich text correctly without altering the source workbook.