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/Findlibgit2.cmake

@@ -0,0 +1,32 @@
+# Findlibgit2.cmake for WASM cross-compilation
+# libgit2 headers downloaded via scripts/deps/build-libgit2-headers.sh
+# No library - git functionality won't work in browser
+
+# Look for headers in CMAKE_PREFIX_PATH (sysroot)
+foreach(_path ${CMAKE_PREFIX_PATH})
+    if(EXISTS "${_path}/include/git2.h")
+        set(LIBGIT2_INCLUDE_DIR "${_path}/include")
+        break()
+    endif()
+endforeach()
+
+set(LIBGIT2_INCLUDE_DIRS "${LIBGIT2_INCLUDE_DIR}")
+set(LIBGIT2_LIBRARY "")  # No actual library - will be stubbed
+set(LIBGIT2_LIBRARIES "")
+set(LIBGIT2_VERSION "1.7.1")
+
+# Check if headers exist
+if(NOT LIBGIT2_INCLUDE_DIR OR NOT EXISTS "${LIBGIT2_INCLUDE_DIR}/git2.h")
+    message(FATAL_ERROR "libgit2 headers not found. Run: ./scripts/deps/build-libgit2-headers.sh")
+endif()
+
+set(LIBGIT2_FOUND TRUE)
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(libgit2
+    REQUIRED_VARS LIBGIT2_INCLUDE_DIR
+    VERSION_VAR LIBGIT2_VERSION)
+
+mark_as_advanced(LIBGIT2_INCLUDE_DIR LIBGIT2_LIBRARY)
+
+message(STATUS "Using libgit2 stub for WASM build (git features disabled)")