Add Excel-style conditional formatting support to AddSheetOptions

[davidnmbond]

Summary

Add Excel-style conditional formatting support to AddSheetOptions.

Scope

• Add AddSheetOptions.ConditionalFormats
• Add public configuration types: ConditionalFormat, ConditionalFormatRule, ConditionalFormatStyle, ConditionalFormatRuleType, ConditionalFormatOperator
• Write OpenXML conditional formatting to generated worksheets
• Support multiple rules per format and multiple target columns
• Allow all-column rules when ColumnNames is omitted

Notes

• The API stays tightly coupled to Excel/OpenXML conditional formatting concepts.
• Column names match the final exported header text.
• Multi-column targets emit per-column conditional-formatting blocks so relative formulas evaluate correctly.

Validation

• Structural OpenXML unit tests cover multiple rules on one column, multiple target columns, and all-column targeting.
• The generated workbook was manually verified in Excel.

Documentation

• README usage examples
• XML documentation examples on the public API
• Work log entry in IMPROVEMENTS.md