- Add comprehensive closure independence tests (69/69 tests pass)
- Validate deepcopy necessity for function inputs with parameter modification
- Document closure capture behavior: references vs values
- Add empirical test for deepcopy necessity investigation
- Update progress and walkthrough documentation
- Confirm deepcopy is required for all 7 occurrences due to closure semantics

Key findings:
- Julia closures capture variable references, not values
- deepcopy essential for isolation against external variable modifications
- Test suite now robustly validates closure independence

- Create discretization_utils.jl with generic _discretize_function
- Add _discretize_dual helper for optional dual functions
- Support TimeGridModel and auto-dimension detection
- Create _serialize_solution for complete solution serialization
- Add comprehensive tests (21/21 pass)
- Use public getters for solution access (best practice)
- Prepare for JLD2 improvement and build_solution refactoring

Key features:
- Single generic function instead of 3 specialized ones
- Proper encapsulation using getters
- All dual variables handled (path, state, control, boundary, variable)
- Compatible with build_solution for reconstruction

- Refactor export_ocp_solution to use _serialize_solution
- Refactor import_ocp_solution to reconstruct via build_solution
- Store discrete data instead of function objects
- Eliminates most JLD2 warnings (only OCP model functions remain)
- Create test/extras/Project.toml for testing scripts
- Add test_jld2_roundtrip.jl script for validation
- Round-trip test successful: all values match perfectly

Benefits:
- Same discretization logic as JSON (code reuse)
- No function serialization warnings for solution data
- Perfect reconstruction via build_solution
- Consistent with JSON export strategy

✅ FINAL VALIDATION COMPLETE
- 3424/3424 tests pass (full suite)
- 1735/1735 serialization tests pass
- 21/21 discretization utils tests pass

📋 PHASE 4 SUMMARY:
1. Extract discretization utilities (discretization_utils.jl)
2. Create _serialize_solution using public getters
3. Refactor JLD2 to use discretization (eliminate warnings)
4. Add comprehensive test infrastructure
5. Validate round-trip JLD2 serialization
6. Confirm no regressions in full test suite

🎯 BENEFITS:
- Single generic discretization function (not 3)
- Code reuse between JSON and JLD2
- No function serialization warnings
- Perfect reconstruction via build_solution
- Improved maintainability and documentation

📊 TEST RESULTS:
- Phase 2: 1726/1726 ✓
- Phase 3: 69/69 ✓
- Phase 4: 21/21 ✓
- Full suite: 3424/3424 ✓

Ready for merge into develop branch.