feat(ooxml): Add conditional formatting support

[arcaputo3]

Summary

Add support for conditional formatting rules.

Syntax

# Highlight cells > 100
xl -f file.xlsx -s Sheet1 -o out.xlsx cond-format B2:B100 \
  --rule ">100" --bg green --fg white

# Data bars
xl -f file.xlsx -s Sheet1 -o out.xlsx cond-format B2:B100 \
  --rule "data-bars" --color blue

# Color scale (gradient)
xl -f file.xlsx -s Sheet1 -o out.xlsx cond-format B2:B100 \
  --rule "color-scale" --min red --max green

# Icon sets
xl -f file.xlsx -s Sheet1 -o out.xlsx cond-format B2:B100 \
  --rule "icon-set" --icons "3-arrows"

Rule Types

1. Cell Value: ">100", "<0", "=X"
2. Data Bars: Visual bars showing relative values
3. Color Scale: Gradient based on values
4. Icon Sets: Arrows, traffic lights, etc.
5. Formulas: Custom formula-based rules

OOXML Implementation

Requires writing <conditionalFormatting> elements to worksheet.xml:

<conditionalFormatting sqref="B2:B100">
  <cfRule type="cellIs" operator="greaterThan" priority="1">
    <formula>100</formula>
    <dxf>
      <fill><patternFill><bgColor rgb="FF00FF00"/></patternFill></fill>
    </dxf>
  </cfRule>
</conditionalFormatting>

Complexity

This is a significant feature requiring:

• New OOXML writer for conditionalFormatting
• Differential formatting (dxf) support
• Multiple rule type handlers
• Priority management for overlapping rules

Acceptance Criteria

• Basic cell value rules work
• Formatting appears correctly in Excel
• Data bars work (simpler case)
• Multiple rules on same range work

Priority

Low - Complex OOXML feature

Milestone

v0.8.0+

Related

• Mentioned in GH#90 (WYSIWYG rendering)