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}"