feat: Add part selector dropdown to IntrospectionPanel

- Fetch available parts from /nx/parts on panel mount
- Dropdown to select which part to introspect (default = assembly)
- Hides idealized parts (*_i.prt) from dropdown
- Shows part size in dropdown (KB or MB)
- Header shows selected part name in primary color
- Refresh button respects current part selection
- Auto-introspects when part selection changes

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

@@ -138,6 +138,15 @@ interface BaselineRunResult {
   message: string;
 }
 
+// Part info from /nx/parts endpoint
+interface PartInfo {
+  name: string;
+  stem: string;
+  is_idealized: boolean;
+  size_kb: number;
+  has_cache: boolean;
+}
+
 export function IntrospectionPanel({ filePath, studyId, onClose }: IntrospectionPanelProps) {
   const [result, setResult] = useState<IntrospectionResult | null>(null);
   const [isLoading, setIsLoading] = useState(false);
@@ -147,14 +156,37 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
   );
   const [searchTerm, setSearchTerm] = useState('');
   
+  // Part selection state
+  const [availableParts, setAvailableParts] = useState<PartInfo[]>([]);
+  const [selectedPart, setSelectedPart] = useState<string>(''); // empty = default/assembly
+  const [isLoadingParts, setIsLoadingParts] = useState(false);
+  
   // Baseline run state
   const [isRunningBaseline, setIsRunningBaseline] = useState(false);
   const [baselineResult, setBaselineResult] = useState<BaselineRunResult | null>(null);
 
   const { addNode, nodes } = useCanvasStore();
 
-  const runIntrospection = useCallback(async () => {
+  // Fetch available parts when studyId changes
-    if (!filePath) return;
+  const fetchAvailableParts = useCallback(async () => {
+    if (!studyId) return;
+    
+    setIsLoadingParts(true);
+    try {
+      const res = await fetch(`/api/optimization/studies/${studyId}/nx/parts`);
+      if (res.ok) {
+        const data = await res.json();
+        setAvailableParts(data.parts || []);
+      }
+    } catch (e) {
+      console.error('Failed to fetch parts:', e);
+    } finally {
+      setIsLoadingParts(false);
+    }
+  }, [studyId]);
+
+  const runIntrospection = useCallback(async (partName?: string) => {
+    if (!filePath && !studyId) return;
 
     setIsLoading(true);
     setError(null);
@@ -163,8 +195,11 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
       
       // If we have a studyId, use the study-aware introspection endpoint
       if (studyId) {
-        // Don't encode studyId - it may contain slashes for nested paths (e.g., M1_Mirror/study_name)
+        // Use specific part endpoint if a part is selected
-        res = await fetch(`/api/optimization/studies/${studyId}/nx/introspect`);
+        const endpoint = partName 
+          ? `/api/optimization/studies/${studyId}/nx/introspect/${encodeURIComponent(partName)}`
+          : `/api/optimization/studies/${studyId}/nx/introspect`;
+        res = await fetch(endpoint);
       } else {
         // Fallback to direct path introspection
         res = await fetch('/api/nx/introspect', {
@@ -191,9 +226,21 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
     }
   }, [filePath, studyId]);
 
+  // Fetch parts list on mount
   useEffect(() => {
-    runIntrospection();
+    fetchAvailableParts();
-  }, [runIntrospection]);
+  }, [fetchAvailableParts]);
+
+  // Run introspection when component mounts or selected part changes
+  useEffect(() => {
+    runIntrospection(selectedPart || undefined);
+  }, [selectedPart]); // eslint-disable-line react-hooks/exhaustive-deps
+  
+  // Handle part selection change
+  const handlePartChange = (e: React.ChangeEvent<HTMLSelectElement>) => {
+    const newPart = e.target.value;
+    setSelectedPart(newPart);
+  };
 
   // Run baseline FEA simulation
   const runBaseline = useCallback(async () => {
@@ -301,11 +348,14 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
       <div className="flex items-center justify-between px-4 py-3 border-b border-dark-700">
         <div className="flex items-center gap-2">
           <Search size={16} className="text-primary-400" />
-          <span className="font-medium text-white text-sm">Model Introspection</span>
+          <span className="font-medium text-white text-sm">
+            Model Introspection
+            {selectedPart && <span className="text-primary-400 ml-1">({selectedPart})</span>}
+          </span>
         </div>
         <div className="flex items-center gap-1">
           <button
-            onClick={runIntrospection}
+            onClick={() => runIntrospection(selectedPart || undefined)}
             disabled={isLoading}
             className="p-1.5 text-dark-400 hover:text-white hover:bg-dark-700 rounded transition-colors"
             title="Refresh"
@@ -321,8 +371,37 @@ export function IntrospectionPanel({ filePath, studyId, onClose }: Introspection
         </div>
       </div>
 
-      {/* Search */}
+      {/* Part Selector + Search */}
-      <div className="px-4 py-2 border-b border-dark-700">
+      <div className="px-4 py-2 border-b border-dark-700 space-y-2">
+        {/* Part dropdown */}
+        {studyId && availableParts.length > 0 && (
+          <div className="flex items-center gap-2">
+            <label className="text-xs text-dark-400 whitespace-nowrap">Part:</label>
+            <select
+              value={selectedPart}
+              onChange={handlePartChange}
+              disabled={isLoading || isLoadingParts}
+              className="flex-1 px-2 py-1.5 bg-dark-800 border border-dark-600 rounded-lg
+                         text-sm text-white focus:outline-none focus:border-primary-500
+                         disabled:opacity-50"
+            >
+              <option value="">Default (Assembly)</option>
+              {availableParts
+                .filter(p => !p.is_idealized) // Hide idealized parts
+                .map(part => (
+                  <option key={part.name} value={part.stem}>
+                    {part.stem} ({part.size_kb > 1000 ? `${(part.size_kb/1024).toFixed(1)}MB` : `${part.size_kb}KB`})
+                  </option>
+                ))
+              }
+            </select>
+            {isLoadingParts && (
+              <RefreshCw size={12} className="animate-spin text-dark-400" />
+            )}
+          </div>
+        )}
+        
+        {/* Search input */}
         <input
           type="text"
           placeholder="Filter expressions..."