Summary

This PR adds a patch system to simplify future expat library upgrades. The src/xmlparse.cpp file contains POCO-specific modifications to expat's original xmlparse.c, and previously these changes had to be manually re-applied after each upgrade.

Changes

New files

• xmlparse_poco.patch - Patch file containing all POCO-specific modifications
• src/xmlparse.c - Original unmodified file from expat 2.7.1 (excluded from build)

POCO-specific modifications captured in the patch

1. EXPAT_POCO mode - Enables POCO-specific entropy generation
2. Poco::RandomInputStream - Uses POCO's random stream instead of OS-specific entropy sources
3. C++ casts - Adds explicit casts for malloc_fcn calls (required for C++ compilation)
4. expat_config.h - Includes the configuration header

CMake changes

• Added version and source URL documentation to CMakeLists.txt
• Exclude xmlparse.c from build (we use xmlparse.cpp)
• Added patch_expat_xmlparse custom target (excluded from ALL)

How to upgrade expat in the future

1. Download new expat release
2. Replace files in dependencies/expat/src/ with new release files
3. Run: cmake --build <build-dir> --target patch_expat_xmlparse
4. The target will apply POCO patches to xmlparse.c and generate xmlparse.cpp