Add board_model library and Phase 3 wxUniversal plan

- Add board_model library with PCB data structures (BOARD, FOOTPRINT, PAD, etc.)
- Add thirdparty include paths (fmt, nlohmann_json, magic_enum, dynamic_bitset)
- Enable wxRegex support in wxBase build (builtin PCRE2)
- Add wxregex imported library to CMakeLists.txt
- Add Phase 3 implementation plan for wxWidgets wxUniversal WASM port

The board_model target includes:
- Common infrastructure (eda_item, eda_shape, eda_text, lset, kiid, etc.)
- PCB board model (board, footprint, pad, pcb_track, zone, etc.)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/build-wxbase-wasm.sh

@@ -42,7 +42,7 @@ emcmake cmake "$WX_SOURCE" \
     -DwxBUILD_TESTS=OFF \
     -DwxBUILD_DEMOS=OFF \
     -DwxBUILD_BENCHMARKS=OFF \
-    -DwxUSE_REGEX=OFF \
+    -DwxUSE_REGEX=builtin \
     -DwxUSE_ZLIB=OFF \
     -DwxUSE_EXPAT=OFF \
     -DwxUSE_LIBJPEG=OFF \