Add SVGExporter with full PAGX-to-SVG conversion support

[OnionsYu]

Summary

• Add SVGExporter class implementing full PAGX-to-SVG conversion, including support for fills, strokes, gradients, image patterns, blend modes, Display P3 color space, shadow filters, color matrix/blend filters, masks, clipPaths, text alignment, and custom data attributes.
• Add export-svg CLI command for exporting PAGX files to SVG format.
• Extract SVG-specific test cases into PAGXSVGTest.cpp and add roundtrip test suite for CLI export.

Key Changes

• New files: SVGExporter.h, SVGExporter.cpp, CommandExportSVG.cpp/.h, PAGXSVGTest.cpp
• Refactored: SVGImporter.cpp (fix pattern import round-trip), PAGXTest.cpp (extract SVG tests), PAGXCliTest.cpp (add CLI roundtrip tests)
• Internal improvements: SVGWriter class encapsulation, SVGBuilder optimizations, shared def ID counter to avoid collisions in nested writers

Test Plan

• All existing PAGX tests pass
• New SVG export roundtrip tests pass
• CLI export-svg command works correctly

Made with Cursor

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.81232% with 521 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.22%. Comparing base (1db9626) to head (4abbaa2).

Files with missing lines	Patch %	Lines
src/pagx/svg/SVGExporter.cpp	71.54%	180 Missing and 55 partials ⚠️
src/pagx/svg/SVGTextLayout.cpp	6.31%	191 Missing and 2 partials ⚠️
src/cli/CommandConvert.cpp	41.84%	68 Missing and 14 partials ⚠️
test/src/PAGXSVGTest.cpp	98.06%	3 Missing and 7 partials ⚠️
src/pagx/svg/SVGImporter.cpp	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3317      +/-   ##
==========================================
- Coverage   75.29%   75.22%   -0.08%     
==========================================
  Files         504      509       +5     
  Lines       33422    35211    +1789     
  Branches    10673    11270     +597     
==========================================
+ Hits        25166    26487    +1321     
- Misses       6032     6424     +392     
- Partials     2224     2300      +76     

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