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/FindZSTD.cmake

@@ -0,0 +1,48 @@
+# FindZSTD.cmake for WASM cross-compilation
+# This override is needed because CMake's find_path/find_library fail during cross-compilation
+
+# ZSTD_INCLUDE_DIR and ZSTD_LIBRARY should be passed from the build script
+# or we can use CMAKE_PREFIX_PATH/ZSTD_ROOT
+
+if(NOT ZSTD_INCLUDE_DIR)
+    find_path(ZSTD_INCLUDE_DIR NAMES zstd.h
+        PATHS ${ZSTD_ROOT} ${CMAKE_PREFIX_PATH}
+        PATH_SUFFIXES include
+        NO_DEFAULT_PATH)
+endif()
+
+if(NOT ZSTD_LIBRARY)
+    find_library(ZSTD_LIBRARY NAMES zstd libzstd zstd_static
+        PATHS ${ZSTD_ROOT} ${CMAKE_PREFIX_PATH}
+        PATH_SUFFIXES lib
+        NO_DEFAULT_PATH)
+endif()
+
+# Fallback: if not found via find_* commands, check if passed directly
+if(NOT ZSTD_INCLUDE_DIR AND DEFINED ENV{SYSROOT})
+    set(ZSTD_INCLUDE_DIR "$ENV{SYSROOT}/include")
+endif()
+if(NOT ZSTD_LIBRARY AND DEFINED ENV{SYSROOT})
+    set(ZSTD_LIBRARY "$ENV{SYSROOT}/lib/libzstd.a")
+endif()
+
+# Verify files exist
+if(ZSTD_INCLUDE_DIR AND NOT EXISTS "${ZSTD_INCLUDE_DIR}/zstd.h")
+    message(WARNING "ZSTD headers not found at ${ZSTD_INCLUDE_DIR}/zstd.h")
+    unset(ZSTD_INCLUDE_DIR)
+endif()
+
+if(ZSTD_LIBRARY AND NOT EXISTS "${ZSTD_LIBRARY}")
+    message(WARNING "ZSTD library not found at ${ZSTD_LIBRARY}")
+    unset(ZSTD_LIBRARY)
+endif()
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(ZSTD DEFAULT_MSG
+    ZSTD_LIBRARY ZSTD_INCLUDE_DIR)
+
+if(ZSTD_FOUND)
+    message(STATUS "Found Zstd: ${ZSTD_LIBRARY}")
+endif()
+
+mark_as_advanced(ZSTD_INCLUDE_DIR ZSTD_LIBRARY)