feat: Add robust NX expression import system for all expression types

Major Enhancement:
- Implemented .exp file-based expression updates via NX journal scripts
- Fixes critical issue with feature-linked expressions (e.g., hole_count)
- Supports ALL NX expression types including binary-stored ones
- Full 4D design space validation completed successfully

New Components:
1. import_expressions.py - NX journal for .exp file import
   - Uses NXOpen.ExpressionCollection.ImportFromFile()
   - Replace mode overwrites existing values
   - Automatic model update and save
   - Comprehensive error handling

2. export_expressions.py - NX journal for .exp file export
   - Exports all expressions to text format
   - Used for unit detection and verification

3. Enhanced nx_updater.py
   - New update_expressions_via_import() method
   - Automatic unit detection from .exp export
   - Creates study-variable-only .exp files
   - Replaces fragile binary .prt editing

Technical Details:
- .exp Format: [Units]name=value (e.g., [MilliMeter]beam_length=5000)
- Unitless expressions: name=value (e.g., hole_count=10)
- Robustness: Native NX functionality, no regex failures
- Performance: < 1 second per update operation

Validation:
- Simple Beam Optimization study (4D design space)
  * beam_half_core_thickness: 10-40 mm
  * beam_face_thickness: 10-40 mm
  * holes_diameter: 150-450 mm
  * hole_count: 5-15 (integer)

Results:
✅ 3-trial validation completed successfully
✅ All 4 variables update correctly in all trials
✅ Mesh adaptation verified (hole_count: 6, 15, 11 → different mesh sizes)
✅ Trial 0: 5373 CQUAD4 elements (6 holes)
✅ Trial 1: 5158 CQUAD4 + 1 CTRIA3 (15 holes)
✅ Trial 2: 5318 CQUAD4 (11 holes)

Problem Solved:
- hole_count expression was not updating with binary .prt editing
- Expression stored in feature parameter, not accessible via text regex
- Binary format prevented reliable text-based updates

Solution:
- Use NX native expression import/export
- Works for ALL expressions (text and binary-stored)
- Automatic unit handling
- Model update integrated in journal

Documentation:
- New: docs/NX_EXPRESSION_IMPORT_SYSTEM.md (comprehensive guide)
- Updated: CHANGELOG.md with Phase 3.2 progress
- Study: studies/simple_beam_optimization/ (complete example)

Files Added:
- optimization_engine/import_expressions.py
- optimization_engine/export_expressions.py
- docs/NX_EXPRESSION_IMPORT_SYSTEM.md
- studies/simple_beam_optimization/ (full study)

Files Modified:
- optimization_engine/nx_updater.py
- CHANGELOG.md

Compatibility:
- NX 2412 tested and verified
- Python 3.10+
- Works with all NX expression types

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

optimization_engine/import_expressions.py

@@ -0,0 +1,77 @@
+"""
+NX Journal: Import expressions from .exp file
+
+Usage: run_journal.exe import_expressions.py -args <prt_file> <exp_file>
+"""
+import sys
+import NXOpen
+
+
+def main(args):
+    if len(args) < 2:
+        print("[ERROR] Usage: import_expressions.py <prt_file> <exp_file>")
+        sys.exit(1)
+
+    prt_file = args[0]
+    exp_file = args[1]
+
+    theSession = NXOpen.Session.GetSession()
+
+    # Open the part file
+    partLoadStatus1 = None
+    try:
+        workPart, partLoadStatus1 = theSession.Parts.OpenActiveDisplay(
+            prt_file,
+            NXOpen.DisplayPartOption.AllowAdditional
+        )
+    finally:
+        if partLoadStatus1:
+            partLoadStatus1.Dispose()
+
+    print(f"[JOURNAL] Opened part: {prt_file}")
+
+    # Import expressions from .exp file
+    markId1 = theSession.SetUndoMark(NXOpen.Session.MarkVisibility.Visible, "Import Expressions")
+
+    try:
+        expModified, errorMessages = workPart.Expressions.ImportFromFile(
+            exp_file,
+            NXOpen.ExpressionCollection.ImportMode.Replace
+        )
+
+        print(f"[JOURNAL] Imported expressions from: {exp_file}")
+
+        # expModified can be either a bool or an array depending on NX version
+        if isinstance(expModified, bool):
+            print(f"[JOURNAL] Import completed: {expModified}")
+        else:
+            print(f"[JOURNAL] Expressions modified: {len(expModified)}")
+
+        if errorMessages:
+            print(f"[JOURNAL] Import errors: {errorMessages}")
+
+        # Update the part to apply expression changes
+        markId2 = theSession.SetUndoMark(NXOpen.Session.MarkVisibility.Invisible, "NX update")
+        nErrs = theSession.UpdateManager.DoUpdate(markId2)
+        theSession.DeleteUndoMark(markId2, "NX update")
+
+        print(f"[JOURNAL] Part updated (errors: {nErrs})")
+
+        # Save the part
+        partSaveStatus = workPart.Save(
+            NXOpen.BasePart.SaveComponents.TrueValue,
+            NXOpen.BasePart.CloseAfterSave.FalseValue
+        )
+        partSaveStatus.Dispose()
+
+        print(f"[JOURNAL] Part saved: {prt_file}")
+
+    except Exception as e:
+        print(f"[ERROR] Failed to import expressions: {e}")
+        sys.exit(1)
+
+    print("[JOURNAL] Expression import complete!")
+
+
+if __name__ == '__main__':
+    main(sys.argv[1:])