feat(ui): edit objective/constraint source in panel + UNSET wiring

atomizer-dashboard/frontend/src/components/canvas/SpecRenderer.tsx

@@ -560,9 +560,10 @@ function SpecRendererInner({
           // clear the target's source to avoid stale runnable config.
           if (sourceType === 'extractor' && (targetType === 'objective' || targetType === 'constraint')) {
             updateNode(edge.target, {
-              // Setting to an empty object would violate schema; clear to placeholder
-              // and let validation catch missing wiring.
-              source: { extractor_id: '', output_name: '' },
+              // Objective/constraint.source is required by schema.
+              // Use explicit UNSET placeholders so validation can catch it
+              // without risking accidental execution.
+              source: { extractor_id: '__UNSET__', output_name: '__UNSET__' },
             }).catch((err) => {
               console.error('Failed to clear source on node:', err);
               setError(err.message);