Summary

• Use unpatched vanilla Expat sources instead of POCO-patched version (fixes Use vanilla Expat sources #5178)
• Upgrade bundled Expat from 2.7.3 to 2.7.4 (fixes Upgrade bundled expat to 2.7.4 #5177)
• Generate expat_config.h at build time using CMake's configure_file()
• Platform-specific entropy source detection (arc4random_buf, getrandom, /dev/urandom)

Changes

• Remove xmlparse.cpp (patched C++ version) and xmlparse_poco.patch
• Remove POCO's RandomInputStream integration - use native entropy sources
• Update both CMakeLists.txt and traditional Makefile
• Expat is now self-contained (no Foundation dependency)

Test plan

• Build succeeds on macOS
• All 138 XML tests pass