feat: Add sub-part introspection and existing FEA results UI

Backend:
- GET /nx/parts - List all .prt files in model directory
- GET /nx/introspect/{part_name} - Introspect a specific part file
  (e.g., M1_Blank.prt instead of just the assembly)
- Each part gets its own cache file (_introspection_{stem}.json)

Frontend IntrospectionPanel:
- Add 'FEA Results' section showing existing OP2/F06 sources
- Green checkmark when results exist, shows recommended source
- Expand file_deps and fea_results sections by default
- Add CheckCircle2 and Database icons

This allows introspecting component parts that contain the actual
design variable expressions (e.g., M1_Blank has 56 expressions
while the assembly ASSY_M1 only has 5).

atomizer-dashboard/frontend/src/components/canvas/panels/IntrospectionPanel.tsx

@@ -21,6 +21,8 @@ import {
   Settings2,
   GitBranch,
   File,
+  Database,
+  CheckCircle2,
 } from 'lucide-react';
 import { useCanvasStore } from '../../../hooks/useCanvasStore';
 
@@ -101,6 +103,22 @@ interface IntrospectionResult {
   units?: Record<string, unknown>;
   linked_parts?: Record<string, unknown>;
   file_dependencies?: FileDependencies;
+  existing_fea_results?: {
+    has_results: boolean;
+    sources: Array<{
+      location: string;
+      path: string;
+      op2: string[];
+      f06: string[];
+      bdf: string[];
+      timestamp?: number;
+    }>;
+    recommended?: {
+      location: string;
+      path: string;
+      op2: string[];
+    };
+  };
 }
 
 // Baseline run result interface
@@ -125,7 +143,7 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
   const [isLoading, setIsLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [expandedSections, setExpandedSections] = useState<Set<string>>(
-    new Set(['expressions', 'extractors'])
+    new Set(['expressions', 'extractors', 'file_deps', 'fea_results'])
   );
   const [searchTerm, setSearchTerm] = useState('');
   
@@ -783,6 +801,77 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
               </div>
             )}
 
+            {/* Existing FEA Results Section */}
+            {result.existing_fea_results && (
+              <div className="border border-dark-700 rounded-lg overflow-hidden">
+                <button
+                  onClick={() => toggleSection('fea_results')}
+                  className="w-full flex items-center justify-between px-3 py-2 bg-dark-800 hover:bg-dark-750 transition-colors"
+                >
+                  <div className="flex items-center gap-2">
+                    {result.existing_fea_results.has_results ? (
+                      <CheckCircle2 size={14} className="text-green-400" />
+                    ) : (
+                      <Database size={14} className="text-dark-500" />
+                    )}
+                    <span className="text-sm font-medium text-white">
+                      FEA Results {result.existing_fea_results.has_results ? 
+                        `(${result.existing_fea_results.sources?.length || 0} sources)` : 
+                        '(none)'}
+                    </span>
+                  </div>
+                  {expandedSections.has('fea_results') ? (
+                    <ChevronDown size={14} className="text-dark-400" />
+                  ) : (
+                    <ChevronRight size={14} className="text-dark-400" />
+                  )}
+                </button>
+
+                {expandedSections.has('fea_results') && (
+                  <div className="p-2 space-y-2">
+                    {result.existing_fea_results.has_results ? (
+                      <>
+                        <p className="text-xs text-green-400 mb-2">
+                          Existing results found - no solve needed for extraction
+                        </p>
+                        {result.existing_fea_results.sources?.map((source, idx) => (
+                          <div 
+                            key={idx} 
+                            className={`p-2 rounded text-xs ${
+                              source.location === result.existing_fea_results?.recommended?.location 
+                                ? 'bg-green-500/10 border border-green-500/30' 
+                                : 'bg-dark-850'
+                            }`}
+                          >
+                            <div className="flex items-center justify-between mb-1">
+                              <span className="text-white font-medium">{source.location}</span>
+                              {source.location === result.existing_fea_results?.recommended?.location && (
+                                <span className="text-[10px] text-green-400 bg-green-500/20 px-1 rounded">
+                                  recommended
+                                </span>
+                              )}
+                            </div>
+                            <div className="text-dark-400 space-y-0.5">
+                              {source.op2?.length > 0 && (
+                                <p>OP2: {source.op2.join(', ')}</p>
+                              )}
+                              {source.f06?.length > 0 && (
+                                <p>F06: {source.f06.join(', ')}</p>
+                              )}
+                            </div>
+                          </div>
+                        ))}
+                      </>
+                    ) : (
+                      <p className="text-xs text-dark-500 text-center py-2">
+                        No FEA results found. Run baseline to generate results.
+                      </p>
+                    )}
+                  </div>
+                )}
+              </div>
+            )}
+
             {/* Extractors Section - only show if available */}
             {(result.extractors_available?.length ?? 0) > 0 && (
             <div className="border border-dark-700 rounded-lg overflow-hidden">