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,
       };