Add PDF export support for PAG files

[OnionsYu]

This PR adds a full-featured PDF exporter for PAG files, including:

• New PDFExporter implementation supporting layers, groups, transforms, blend modes, gradients, patterns, images, soft masks, and text glyphs
• CID font embedding with UTF-16BE encoding for non-ASCII text rendering
• Shared ExporterUtils extracted from SVG and PDF exporters to reduce code duplication
• New cmake option PAG_BUILD_PDF to control PDF export feature inclusion
• New test cases covering PDF export functionality

The exporter converts PAG content to PDF format by traversing the layer tree and generating corresponding PDF drawing operations.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.63319% with 130 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.16%. Comparing base (7395eb2) to head (7c6a302).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pagx/pdf/PDFExporter.cpp	89.07%	59 Missing and 45 partials ⚠️
src/pagx/utils/ExporterUtils.cpp	94.28%	5 Missing and 5 partials ⚠️
test/src/PAGXPDFTest.cpp	99.12%	3 Missing and 6 partials ⚠️
src/cli/CommandConvert.cpp	90.24%	2 Missing and 2 partials ⚠️
test/src/PAGXSVGTest.cpp	99.47%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3343      +/-   ##
==========================================
+ Coverage   75.21%   77.16%   +1.94%     
==========================================
  Files         509      513       +4     
  Lines       35211    38062    +2851     
  Branches    11270    11709     +439     
==========================================
+ Hits        26485    29369    +2884     
+ Misses       6422     6331      -91     
- Partials     2304     2362      +58     

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