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

atomizer-dashboard/frontend/src/lib/validation/specValidator.ts

@@ -178,9 +178,14 @@ const validationRules: ValidationRule[] = [
           edge => edge.target === obj.id && edge.source.startsWith('ext_')
         );
         
-        // Also check if source.extractor_id is set
-        const hasDirectSource = obj.source?.extractor_id && 
-          spec.extractors.some(e => e.id === obj.source.extractor_id);
+        // Also check if source.extractor_id is set (and not UNSET placeholders)
+        const extractorId = obj.source?.extractor_id;
+        const outputName = obj.source?.output_name;
+        const hasDirectSource = Boolean(extractorId) &&
+          extractorId !== '__UNSET__' &&
+          Boolean(outputName) &&
+          outputName !== '__UNSET__' &&
+          spec.extractors.some(e => e.id === extractorId);
         
         if (!hasSource && !hasDirectSource) {
           return {