Fix PHP warning for missing runtime.asset.php in production builds

[Copilot]

In production builds, build/runtime.asset.php doesn't exist, but the code unconditionally tried to include it, generating noisy PHP warnings. The HMR runtime file should only be loaded when both the dependency is declared and the file is present.

Changes

• inc/namespace.php — Reorder and tighten the runtime asset guard in enqueue_assets_for_post:
  1. Check wp-react-refresh-runtime is in the dependency array first (cheap, no I/O)
  2. Verify runtime.asset.php is readable on disk via is_readable() before attempting include
  3. Only then include the file and register the HMR runtime script

// Before — unconditionally includes, warns if file absent
$runtime_asset = include plugin_dir_path( __DIR__ ) . 'build/runtime.asset.php';
if ( ! empty( $runtime_asset ) && in_array( 'wp-react-refresh-runtime', $editor_asset['dependencies'] ?? [], true ) ) {

// After — checks dependency and file existence first
if ( in_array( 'wp-react-refresh-runtime', $editor_asset['dependencies'] ?? [], true ) ) {
    $runtime_asset_file = plugin_dir_path( __DIR__ ) . 'build/runtime.asset.php';
    if ( is_readable( $runtime_asset_file ) ) {
        $runtime_asset = include $runtime_asset_file;
        ...
    }
}