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>

CHANGELOG.md

@@ -6,10 +6,29 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
-### Phase 2 - LLM Integration (In Progress)
+### Phase 3.2 - Integration & NX Enhancements (In Progress)
+
+#### Added
+- **NX Expression Import System** (2025-11-17)
+  - Robust .exp file-based expression updates via NX journal scripts
+  - Supports ALL NX expression types including binary-stored ones
+  - Automatic unit detection and formatting
+  - Fixes issue with `hole_count` and other feature-linked expressions
+  - Documentation: [NX_EXPRESSION_IMPORT_SYSTEM.md](docs/NX_EXPRESSION_IMPORT_SYSTEM.md)
+  - New journal: `import_expressions.py` for .exp file import
+  - Enhanced `nx_updater.py` with `update_expressions_via_import()` method
+
+- **4D Design Space Validation** (2025-11-17)
+  - Validated full 4-variable optimization (beam_half_core_thickness, beam_face_thickness, holes_diameter, hole_count)
+  - All variables now updating correctly in optimization loop
+  - Mesh adaptation verified across different hole_count values
+
+### Phase 2 - LLM Integration (Completed 85%)
 - Natural language interface for optimization configuration
 - Feature registry with capability catalog
 - Claude skill for Atomizer navigation
+- LLM workflow analyzer and extractor orchestration
+- Dynamic code generation for hooks and extractors
 
 ---