fix: Complete stress extraction fix for NX Nastran OP2 files

THREE critical fixes applied:

1. API Access Pattern
   - Support dotted attribute names (e.g., 'stress.chexa_stress')
   - Compatible with newer pyNastran versions (NX 2412.5)
   - Fallback to older API formats for compatibility

2. Correct Von Mises Index
   - Solid elements (CHEXA, CTETRA, CPENTA): index 9
   - Shell elements (CQUAD4, CTRIA3): last column
   - Data structure: [oxx, oyy, ozz, txy, tyz, txz, o1, o2, o3, von_mises]

3. Units Conversion (CRITICAL)
   - NX Nastran outputs stress in kPa, not MPa
   - Apply conversion: kPa / 1000 = MPa
   - Example: 113094.73 kPa -> 113.09 MPa

Test Results:
- Before: 0.00 MPa (FAIL)
- After:  113.09 MPa at element 83 (SUCCESS)

Files modified:
- optimization_engine/result_extractors/op2_extractor_example.py

Test files added:
- examples/test_stress_direct.py
- examples/test_stress_fix.py
- examples/debug_op2_stress.py
- STRESS_EXTRACTION_FIXED.md
- TESTING_STRESS_FIX.md

examples/test_stress_direct.py

@@ -0,0 +1,56 @@
+"""
+Direct test of stress extraction without using cached imports.
+"""
+
+from pathlib import Path
+import sys
+
+# Force reload
+project_root = Path(__file__).parent.parent
+sys.path.insert(0, str(project_root))
+
+# Import directly from the file
+import importlib.util
+spec = importlib.util.spec_from_file_location(
+    "op2_extractor",
+    project_root / "optimization_engine/result_extractors/op2_extractor_example.py"
+)
+op2_extractor = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(op2_extractor)
+
+if __name__ == "__main__":
+    op2_path = project_root / "examples/bracket/bracket_sim1-solution_1.op2"
+
+    print("="*60)
+    print("DIRECT STRESS EXTRACTION TEST")
+    print("="*60)
+    print(f"OP2 file: {op2_path}")
+    print()
+
+    # Test stress extraction
+    print("--- Testing extract_max_stress() ---")
+    try:
+        result = op2_extractor.extract_max_stress(op2_path, stress_type='von_mises')
+        print()
+        print("RESULT:")
+        for key, value in result.items():
+            print(f"  {key}: {value}")
+
+        if result['max_stress'] > 100.0:
+            print()
+            print("SUCCESS! Stress extraction working!")
+            print(f"Got: {result['max_stress']:.2f} MPa")
+        elif result['max_stress'] == 0.0:
+            print()
+            print("FAIL: Still returning 0.0")
+        else:
+            print()
+            print(f"Got unexpected value: {result['max_stress']:.2f} MPa")
+
+    except Exception as e:
+        print(f"ERROR: {e}")
+        import traceback
+        traceback.print_exc()
+
+    print()
+    print("="*60)