Add two-phase build with host asyncify transformation

- Move asyncify from Docker to host to avoid memory issues
- Auto-download Binaryen v121 (v125 has regression bug)
- Use -O1 for debug builds (V8 local count limit)
- Remove asyncify flags from linker (handled by wasm-opt)
- Document two-phase build in build.md

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/common/get-wasm-opt.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+# Download and cache Binaryen wasm-opt
+#
+# Usage: ./scripts/tools/get-wasm-opt.sh
+# Output: Prints path to wasm-opt executable
+#
+# Binaryen v121 is used because v125 has a regression causing asyncify crashes.
+# The binary is cached in tools/binaryen-121/
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "${SCRIPT_DIR}/../.." && pwd)"
+
+BINARYEN_VERSION="121"
+BINARYEN_DIR="${PROJECT_ROOT}/tools/binaryen-${BINARYEN_VERSION}"
+WASM_OPT="${BINARYEN_DIR}/bin/wasm-opt"
+
+download_binaryen() {
+    # Detect platform
+    local os=$(uname -s)
+    local arch=$(uname -m)
+    local platform=""
+
+    case "${os}-${arch}" in
+        Darwin-arm64)  platform="arm64-macos" ;;
+        Darwin-x86_64) platform="x86_64-macos" ;;
+        Linux-aarch64) platform="aarch64-linux" ;;
+        Linux-x86_64)  platform="x86_64-linux" ;;
+        *)
+            echo "ERROR: Unsupported platform: ${os}-${arch}" >&2
+            exit 1
+            ;;
+    esac
+
+    local url="https://github.com/WebAssembly/binaryen/releases/download/version_${BINARYEN_VERSION}/binaryen-version_${BINARYEN_VERSION}-${platform}.tar.gz"
+    local tarball="${PROJECT_ROOT}/tools/binaryen-${BINARYEN_VERSION}.tar.gz"
+
+    echo "Downloading Binaryen v${BINARYEN_VERSION} for ${platform}..." >&2
+    mkdir -p "${PROJECT_ROOT}/tools"
+    curl -L -o "${tarball}" "${url}"
+
+    echo "Extracting..." >&2
+    tar -xzf "${tarball}" -C "${PROJECT_ROOT}/tools"
+    mv "${PROJECT_ROOT}/tools/binaryen-version_${BINARYEN_VERSION}" "${BINARYEN_DIR}"
+    rm "${tarball}"
+
+    echo "Binaryen v${BINARYEN_VERSION} installed to ${BINARYEN_DIR}" >&2
+}
+
+# Download Binaryen if not cached
+if [ ! -x "${WASM_OPT}" ]; then
+    download_binaryen
+fi
+
+# Verify version
+INSTALLED_VERSION=$("${WASM_OPT}" --version 2>&1 | grep -o '[0-9]\+' | head -1)
+if [ "${INSTALLED_VERSION}" != "${BINARYEN_VERSION}" ]; then
+    echo "WARNING: wasm-opt version mismatch (got ${INSTALLED_VERSION}, expected ${BINARYEN_VERSION})" >&2
+    echo "Re-downloading..." >&2
+    rm -rf "${BINARYEN_DIR}"
+    download_binaryen
+fi
+
+# Output path to wasm-opt (this is the only stdout output)
+echo "${WASM_OPT}"

scripts/kicad/build-pcbnew.sh

@@ -119,16 +119,22 @@ log_info "Building KiCad PCBnew ${KICAD_VERSION} for WASM..."
 # Use environment DEBUG_BUILD if set, otherwise check local --debug flag
 # -fexceptions is required because wxWidgets is built with exceptions enabled
 # -matomics -mbulk-memory are required for shared memory (pthreads)
+# NOTE: We use -O1 for debug builds because -O0 produces WASM with too many
+# locals for V8/Chrome to compile (error: "local count too large").
+# -O1 keeps debug info but optimizes enough to stay under V8's limits.
 if [ "${DEBUG_BUILD:-0}" = "1" ] || [ $DEBUG -eq 1 ]; then
     BUILD_TYPE="Debug"
-    EXTRA_FLAGS="-g -O0 -fexceptions -matomics -mbulk-memory"
-    LINKER_DEBUG_FLAGS="-g -gsource-map -fexceptions"
-    log_info "Building KiCad in DEBUG mode (with source maps)"
+    EXTRA_FLAGS="-g -O1 -fexceptions -matomics -mbulk-memory"
+    # -O0 at link time skips wasm-opt (which can OOM on large WASM with debug symbols)
+    LINKER_DEBUG_FLAGS="-O0 -g -gsource-map -fexceptions"
+    log_info "Building KiCad in DEBUG mode (with source maps, -O1 for WASM compatibility)"
 else
     BUILD_TYPE="Release"
     EXTRA_FLAGS="-O2 -fexceptions -matomics -mbulk-memory"
-    LINKER_DEBUG_FLAGS="-fexceptions"
-    log_info "Building KiCad in RELEASE mode"
+    # -O0 at link time skips wasm-opt (which can OOM on large WASM files)
+    # Compilation is still -O2 for optimized code, but we skip post-link wasm-opt
+    LINKER_DEBUG_FLAGS="-O0 -fexceptions"
+    log_info "Building KiCad in RELEASE mode (skipping wasm-opt due to memory limits)"
 fi
 
 # Step 6: Create build directory
@@ -178,7 +184,7 @@ emcmake cmake "${KICAD_DIR}" \
     -DCMAKE_POLICY_VERSION_MINIMUM=3.5 \
     -DCMAKE_CXX_FLAGS="${EXTRA_FLAGS} -pthread -sUSE_ZLIB=1 -DKICAD_USE_PLATFORM_WASM=1 -I${SYSROOT}/include" \
     -DCMAKE_C_FLAGS="${EXTRA_FLAGS} -pthread -sUSE_ZLIB=1 -I${SYSROOT}/include" \
-    -DCMAKE_EXE_LINKER_FLAGS="${LINKER_DEBUG_FLAGS} -pthread -sUSE_ZLIB=1 -sASYNCIFY=1 -sASYNCIFY_STACK_SIZE=65536 -sUSE_PTHREADS=1 -sPTHREAD_POOL_SIZE=4 -sALLOW_MEMORY_GROWTH=1 -sINITIAL_MEMORY=256MB -sMAXIMUM_MEMORY=4GB -L${SYSROOT}/lib ${STUBS_BUILD}/libgit2_stub.a ${STUBS_BUILD}/libcurl_stub.a" \
+    -DCMAKE_EXE_LINKER_FLAGS="${LINKER_DEBUG_FLAGS} -pthread -sUSE_ZLIB=1 -sUSE_PTHREADS=1 -sPTHREAD_POOL_SIZE=4 -sALLOW_MEMORY_GROWTH=1 -sINITIAL_MEMORY=256MB -sMAXIMUM_MEMORY=4GB -L${SYSROOT}/lib ${STUBS_BUILD}/libgit2_stub.a ${STUBS_BUILD}/libcurl_stub.a" \
     -DCMAKE_PREFIX_PATH="${SYSROOT};${WX_BUILD}" \
     -DwxWidgets_CONFIG_EXECUTABLE="${WX_BUILD}/wx-config" \
     \