Add KiCad PCBnew build script and CMake Find modules

- Add scripts/kicad/build-pcbnew.sh with clean-by-default and --no-clean flag
- Remove old scripts/build-pcbnew-wasm.sh (moved to scripts/kicad/)
- Add Boost build script with Emscripten toolset for Boost.Locale
- Add header download scripts for CURL and libgit2 (stub libraries)
- Add CMake Find modules for cross-compilation compatibility:
  - FindGLM.cmake: Reads version from setup.hpp
  - FindZSTD.cmake: Explicit sysroot paths
  - FindCURL.cmake: Header-only stub
  - FindSPNAV.cmake: Disabled (3D mouse not needed in browser)
  - FindCairo.cmake: Finds our built Cairo
  - FindPixman.cmake: Finds our built Pixman
  - Findlibgit2.cmake: Header-only stub
- Fix OpenCASCADE build: correct extracted dir name, add CMake policy
- Fix Protobuf build: correct GitHub release tag format (v21.12 not v3.21.12)

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

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

wasm/cmake/FindPixman.cmake

@@ -0,0 +1,57 @@
+# FindPixman.cmake for WASM cross-compilation
+# Pixman was built via scripts/deps/build-pixman.sh
+
+# Look for Pixman in CMAKE_PREFIX_PATH (sysroot)
+foreach(_path ${CMAKE_PREFIX_PATH})
+    if(EXISTS "${_path}/include/pixman-1/pixman.h")
+        set(PIXMAN_INCLUDE_DIR "${_path}/include/pixman-1")
+        break()
+    endif()
+endforeach()
+
+foreach(_path ${CMAKE_PREFIX_PATH})
+    if(EXISTS "${_path}/lib/libpixman-1.a")
+        set(PIXMAN_LIBRARY "${_path}/lib/libpixman-1.a")
+        break()
+    endif()
+endforeach()
+
+# Verify they exist
+if(NOT PIXMAN_INCLUDE_DIR OR NOT EXISTS "${PIXMAN_INCLUDE_DIR}/pixman.h")
+    message(FATAL_ERROR "Pixman headers not found. Run: ./scripts/deps/build-pixman.sh")
+endif()
+
+if(NOT PIXMAN_LIBRARY OR NOT EXISTS "${PIXMAN_LIBRARY}")
+    message(FATAL_ERROR "Pixman library not found. Run: ./scripts/deps/build-pixman.sh")
+endif()
+
+# Extract version from pixman-version.h
+file(STRINGS "${PIXMAN_INCLUDE_DIR}/pixman-version.h" _version_major
+    REGEX "#define[ \t]+PIXMAN_VERSION_MAJOR")
+file(STRINGS "${PIXMAN_INCLUDE_DIR}/pixman-version.h" _version_minor
+    REGEX "#define[ \t]+PIXMAN_VERSION_MINOR")
+file(STRINGS "${PIXMAN_INCLUDE_DIR}/pixman-version.h" _version_micro
+    REGEX "#define[ \t]+PIXMAN_VERSION_MICRO")
+
+if(_version_major AND _version_minor AND _version_micro)
+    string(REGEX MATCH "[0-9]+" _major ${_version_major})
+    string(REGEX MATCH "[0-9]+" _minor ${_version_minor})
+    string(REGEX MATCH "[0-9]+" _micro ${_version_micro})
+    set(PIXMAN_VERSION "${_major}.${_minor}.${_micro}")
+else()
+    set(PIXMAN_VERSION "0.42.2")  # Fallback
+endif()
+
+# Set output variables
+set(PIXMAN_FOUND TRUE)
+set(PIXMAN_INCLUDE_DIRS "${PIXMAN_INCLUDE_DIR}")
+set(PIXMAN_LIBRARIES "${PIXMAN_LIBRARY}")
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(Pixman
+    REQUIRED_VARS PIXMAN_LIBRARY PIXMAN_INCLUDE_DIR
+    VERSION_VAR PIXMAN_VERSION)
+
+mark_as_advanced(PIXMAN_INCLUDE_DIR PIXMAN_LIBRARY)
+
+message(STATUS "Found Pixman for WASM: ${PIXMAN_LIBRARY}")