Add test cases for SVG export, text rendering, and CLI convert command.

[OnionsYu]

新增 SVG 导出功能的测试覆盖，包括以下三个方面：

1. SVG 导出测试（PAGXSVGTest）：新增针对滤镜效果（模糊、投影、颜色矩阵、渐变等）和边界条件（空图层、大尺寸内容、多帧序列等）的综合测试用例。

2. 文字渲染测试：新增 SVGTextLayout 布局测试和 ComputeGlyphPaths 字形路径计算的单元测试。

3. CLI 转换命令测试（PAGXCliTest）：新增针对错误处理（无效输入、文件格式不支持等）和 SVG↔PAGX 双向转换 roundtrip 的综合测试用例。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.94%. Comparing base (7395eb2) to head (43873f0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3338      +/-   ##
==========================================
+ Coverage   75.21%   77.94%   +2.72%     
==========================================
  Files         509      509              
  Lines       35211    37594    +2383     
  Branches    11270    11274       +4     
==========================================
+ Hits        26485    29303    +2818     
+ Misses       6422     5998     -424     
+ Partials     2304     2293      -11     

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