feat: add BorderStyle and IconPrefix decoration to heading styles

[forest6511]

Summary

• Add BorderStyle property to heading styles (single, double, thick, dotted, dashed, dotDash, wave, triple)
• Add IconPrefix (decorative character prepended to heading text) and IconPrefixColor (optional custom color for the icon, falls back to heading color)
• Both single-paragraph and border-extent-text (spacer) paths updated
• 10 new unit tests added (297 total)

YAML Usage

H1:
  showBorder: true
  borderStyle: "double"
  borderPosition: "bottom"

H2:
  iconPrefix: "◆"
  iconPrefixColor: "E91E8C"

Test plan

• Build passes with no errors
• 297 tests passing (10 new tests for BorderStyle and IconPrefix)
• BorderStyle applies to both AddHeadingSingleParagraph and AddHeadingWithSpacers
• IconPrefix prepends a separate run with correct color fallback logic
• StyleApplicator correctly maps all three new properties from config to model

Closes #66

[codecov]

Codecov Report

❌ Patch coverage is 72.50000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (eb706b1) to head (48ef1ff).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...kdownToDocx.Core/OpenXml/OpenXmlDocumentBuilder.cs	63.33%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   89.06%   88.54%   -0.53%     
==========================================
  Files          27       27              
  Lines        1372     1405      +33     
  Branches      174      179       +5     
==========================================
+ Hits         1222     1244      +22     
- Misses        113      122       +9     
- Partials       37       39       +2     

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