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>

scripts/deps/build-libgit2-headers.sh

@@ -0,0 +1,58 @@
+#!/bin/bash
+# Download libgit2 headers for WebAssembly builds
+# We only need the headers - git functionality won't work in browser
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "${SCRIPT_DIR}/../common/env.sh"
+source "${SCRIPT_DIR}/../common/versions.sh"
+source "${SCRIPT_DIR}/../common/functions.sh"
+
+LIBGIT2_VERSION="${LIBGIT2_VERSION:-1.7.1}"
+LIBGIT2_DIR="${DEPS_ROOT}/libgit2-${LIBGIT2_VERSION}"
+LIBGIT2_STAMP="${BUILD_ROOT}/stamps/libgit2-headers.stamp"
+
+# Parse arguments
+CLEAN=0
+for arg in "$@"; do
+    case $arg in
+        --clean)
+            CLEAN=1
+            shift
+            ;;
+    esac
+done
+
+if [ $CLEAN -eq 1 ]; then
+    log_info "Cleaning libgit2 headers..."
+    rm -rf "${LIBGIT2_DIR}" "${LIBGIT2_STAMP}"
+    rm -rf "${SYSROOT}/include/git2.h" "${SYSROOT}/include/git2"
+fi
+
+# Check if already installed
+if check_stamp "${LIBGIT2_STAMP}"; then
+    log_info "libgit2 headers already installed, skipping..."
+    exit 0
+fi
+
+# Download if needed
+if [ ! -d "${LIBGIT2_DIR}" ]; then
+    log_info "Downloading libgit2 ${LIBGIT2_VERSION} headers..."
+    mkdir -p "${DEPS_ROOT}"
+    cd "${DEPS_ROOT}"
+
+    LIBGIT2_URL="https://github.com/libgit2/libgit2/archive/refs/tags/v${LIBGIT2_VERSION}.tar.gz"
+    download_file "${LIBGIT2_URL}" "libgit2-${LIBGIT2_VERSION}.tar.gz"
+    tar -xzf "libgit2-${LIBGIT2_VERSION}.tar.gz"
+    rm "libgit2-${LIBGIT2_VERSION}.tar.gz"
+fi
+
+# Install headers only
+log_info "Installing libgit2 headers to sysroot..."
+mkdir -p "${SYSROOT}/include/git2/sys"
+cp "${LIBGIT2_DIR}/include/git2.h" "${SYSROOT}/include/"
+cp -r "${LIBGIT2_DIR}/include/git2/"* "${SYSROOT}/include/git2/"
+
+create_stamp "${LIBGIT2_STAMP}"
+log_info "libgit2 headers installed!"