Add macOS x86_64 platform support

Platform detection now happens in framework/platform, sourced by both
sync.sh and the node shim. Uses shasum on macOS, sha256sum on Linux.

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

framework/shims/node.common

@@ -7,6 +7,15 @@ project_root="$node_common_DIR/../.."
 # shellcheck source=../versions
 source "$node_common_DIR"/../versions
 
+# shellcheck source=../platform
+source "$node_common_DIR"/../platform
+
+# Get platform-specific node directory
+nodejs_dirname_var="nodejs_dirname_${platform}"
+nodejs_dirname="${!nodejs_dirname_var}"
+nodejs_dist_dir="framework/binaries/$nodejs_dirname"
+nodejs_bin_dir="$nodejs_dist_dir/bin"
+
 nodejs_binary_dir="$project_root/$nodejs_bin_dir"
 
 # This might be too restrictive.  Or not restrictive enough.