Layout refresh latency bug revised

tests/layout-backend-static.test.js

@@ -60,6 +60,18 @@ assert(
   serverPy.includes('svg_url'),
   'save-layout response should include an svg_url for the new layout tab'
 );
+assert(
+  serverPy.includes('svg_ready') &&
+    serverPy.includes('svg_version') &&
+    serverPy.includes('file_version(svg_path)') &&
+    serverPy.includes("url_for('get_layout_svg', project_name=project, cell_name=cell, v=svg_version)"),
+  'save-layout response should only expose a versioned SVG URL after the preview file is ready'
+);
+assert(
+  serverPy.includes('temp_svg_path') &&
+    serverPy.includes('os.replace(temp_svg_path, svg_path)'),
+  'save-layout should publish generated SVG previews atomically instead of serving partially written files'
+);
 assert(
   serverPy.includes('RouterStackUnavailable') &&
     serverPy.includes('except RouterStackUnavailable as e') &&