feat(canvas): Add NX model introspection API endpoints

Phase 4 of Canvas Professional Upgrade:
- Add /studies/{id}/nx/introspect endpoint for full model introspection
- Add /studies/{id}/nx/expressions endpoint for expression list
- Add caching to avoid re-running NX journal on each request
- Add frontend API client methods: introspectNxModel, getNxExpressions
- Use existing introspect_part.py extractor

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

atomizer-dashboard/frontend/src/api/client.ts

@@ -348,6 +348,55 @@ class ApiClient {
     if (!response.ok) throw new Error(`Failed to export ${format}`);
     return response.json();
   }
+  // NX Model introspection
+  async introspectNxModel(studyId: string, force: boolean = false): Promise<{
+    study_id: string;
+    cached: boolean;
+    introspection: {
+      success: boolean;
+      part_file: string;
+      expressions: {
+        user: Array<{
+          name: string;
+          value: number;
+          units?: string;
+          formula?: string;
+        }>;
+        internal: any[];
+        user_count: number;
+        total_count: number;
+      };
+      mass_properties: {
+        mass_kg: number;
+        mass_g: number;
+        volume_mm3: number;
+        surface_area_mm2: number;
+        center_of_gravity_mm: [number, number, number];
+      };
+    };
+  }> {
+    const url = force
+      ? `${API_BASE}/optimization/studies/${studyId}/nx/introspect?force=true`
+      : `${API_BASE}/optimization/studies/${studyId}/nx/introspect`;
+    const response = await fetch(url);
+    if (!response.ok) throw new Error('Failed to introspect NX model');
+    return response.json();
+  }
+
+  async getNxExpressions(studyId: string): Promise<{
+    study_id: string;
+    expressions: Array<{
+      name: string;
+      value: number;
+      units?: string;
+      formula?: string;
+    }>;
+    count: number;
+  }> {
+    const response = await fetch(`${API_BASE}/optimization/studies/${studyId}/nx/expressions`);
+    if (!response.ok) throw new Error('Failed to get NX expressions');
+    return response.json();
+  }
 }
 
 export const apiClient = new ApiClient();