fix(canvas): Multiple fixes for drag-drop, undo/redo, and code generation

Drag-drop fixes:
- Fix Objective default data: use nested 'source' object with extractor_id/output_name
- Fix Constraint default data: use 'type' field (not constraint_type), 'threshold' (not limit)

Undo/Redo fixes:
- Remove dependency on isDirty flag (which is always false due to auto-save)
- Record snapshots based on actual spec changes via deep comparison

Code generation improvements:
- Update system prompt to support multiple extractor types:
  * OP2-based extractors for FEA results (stress, displacement, frequency)
  * Expression-based extractors for NX model values (dimensions, volumes)
  * Computed extractors for derived values (no FEA needed)
- Claude will now choose appropriate signature based on user's description

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

@@ -104,18 +104,24 @@ function getDefaultNodeData(type: AddableNodeType, position: { x: number; y: num
         name: `objective_${timestamp}`,
         direction: 'minimize',
         weight: 1.0,
-        source_extractor_id: null,
-        source_output: null,
+        // Source is required - use placeholder that user must configure
+        source: {
+          extractor_id: 'ext_001',  // Placeholder - user needs to configure
+          output_name: 'value',
+        },
         canvas_position: position,
       };
     case 'constraint':
       return {
         name: `constraint_${timestamp}`,
-        constraint_type: 'hard',  // Must be 'hard' or 'soft'
+        type: 'hard',  // Must be 'hard' or 'soft' (field is 'type' not 'constraint_type')
         operator: '<=',
-        limit: 1.0,
-        source_extractor_id: null,
-        source_output: null,
+        threshold: 1.0,  // Field is 'threshold' not 'limit'
+        // Source is required
+        source: {
+          extractor_id: 'ext_001',  // Placeholder - user needs to configure
+          output_name: 'value',
+        },
         enabled: true,
         canvas_position: position,
       };

atomizer-dashboard/frontend/src/hooks/useSpecUndoRedo.ts

@@ -16,7 +16,7 @@
 
 import { useEffect, useRef } from 'react';
 import { useUndoRedo, UndoRedoResult } from './useUndoRedo';
-import { useSpecStore, useSpec, useSpecIsDirty } from './useSpecStore';
+import { useSpecStore, useSpec } from './useSpecStore';
 import { AtomizerSpec } from '../types/atomizer-spec';
 
 const STORAGE_KEY_PREFIX = 'atomizer-spec-history-';
@@ -28,7 +28,6 @@ export interface SpecUndoRedoResult extends UndoRedoResult<AtomizerSpec | null>
 
 export function useSpecUndoRedo(): SpecUndoRedoResult {
   const spec = useSpec();
-  const isDirty = useSpecIsDirty();
   const studyId = useSpecStore((state) => state.studyId);
   const lastSpecRef = useRef<AtomizerSpec | null>(null);
 
@@ -56,13 +55,21 @@ export function useSpecUndoRedo(): SpecUndoRedoResult {
     },
   });
 
-  // Record snapshot when spec changes (and is dirty)
+  // Record snapshot when spec changes
+  // Note: We removed the isDirty check because with auto-save, isDirty is always false
+  // after the API call completes. Instead, we compare the spec directly.
   useEffect(() => {
-    if (spec && isDirty && spec !== lastSpecRef.current) {
-      lastSpecRef.current = spec;
-      undoRedo.recordSnapshot();
+    if (spec && spec !== lastSpecRef.current) {
+      // Deep compare to avoid recording duplicate snapshots
+      const specStr = JSON.stringify(spec);
+      const lastStr = lastSpecRef.current ? JSON.stringify(lastSpecRef.current) : '';
+      
+      if (specStr !== lastStr) {
+        lastSpecRef.current = spec;
+        undoRedo.recordSnapshot();
+      }
     }
-  }, [spec, isDirty, undoRedo]);
+  }, [spec, undoRedo]);
 
   // Clear history when study changes
   useEffect(() => {