Add mix-blend-mode support to SVG importer.

[OnionsYu]

SVG 元素的 mix-blend-mode 属性此前未被解析，导致导入 SVG 文件时图层混合模式始终为 Normal，与原始设计效果不符。

本次修改新增 SVGBlendMode.h，提供 SVG 混合模式字符串与 BlendMode 枚举之间的双向转换，并在 SVGImporter 解析图层属性时读取 mix-blend-mode 并赋值给对应图层的 blendMode 字段。

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.24%. Comparing base (fd66f78) to head (de03ef3).

Files with missing lines	Patch %	Lines
src/pagx/svg/SVGBlendMode.h	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3334      +/-   ##
==========================================
- Coverage   75.27%   75.24%   -0.03%     
==========================================
  Files         503      504       +1     
  Lines       33414    33422       +8     
  Branches    10668    10673       +5     
==========================================
- Hits        25151    25150       -1     
- Misses       6035     6038       +3     
- Partials     2228     2234       +6     

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