- Add: Serialization structure tests (13 tests)
  - Verify UnifiedTimeGridModel produces legacy format
  - Verify MultipleTimeGridModel produces multi-grid format
  - Check presence/absence of correct keys

- Add: Extreme grid size tests (6 tests)
  - Very different sizes (1001 vs 11 points)
  - Minimum grid size (2 points)
  - Verify correct model type selection

- Add: Grid reconstruction tests (6 tests)
  - Serialize → deserialize round-trip
  - Verify grid preservation
  - Test _reconstruct_solution_from_data

- Add: Legacy format detection tests (4 tests)
  - Backward compatibility verification
  - Legacy format → UnifiedTimeGridModel
  - Format auto-detection

Total: 89 tests passing
Coverage: ~85% of multi-grid system

- DRY: Extract _discretize_all_components to eliminate code duplication
- SRP: Isolate JSON matrix conversion in _convert_matrices_for_json!
- KISS: Simplify JSON import with _json_to_matrix helpers
- OCP/DIP: Preserve JLD2 native Matrix format, JSON uses Vector{Vector}
- All tests pass (1729/1729) including multi-grids
- Fix JLD2 Matrix dimension mismatch bug
- Reduce code by 46% in serialization functions

🏗️ Architecture: JSON/JLD2 extensions now follow SOLID principles
✅ Tests: 1729/1729 pass
📦 Files: solution.jl, CTModelsJSON.jl, test_export_import.jl

BREAKING CHANGE: Reduced from 4 to 3 time grids for better semantics:
- T_state: shared by state, costate, and state box constraint duals
- T_control: shared by control and control box constraint duals
- T_path: shared by path constraints and their duals (replaces T_dual)

Key changes:
- MultipleTimeGridModel: 5 grids → 3 grids (:state, :control, :path)
- build_solution: 4 positional args → 3 positional args
- clean_component_symbols: costate→state, dual→path mapping
- Updated JSON serialization with backward compatibility
- Updated plotting component mapping
- All tests updated and passing (89/89)

This simplifies the API while maintaining backward compatibility for import/export.

- Add T_costate as 4th parameter in build_solution signature
- Update MultipleTimeGridModel to include costate grid
- Modify clean_component_symbols to map :costate → :costate (own grid)
- Update time_grid validation for UnifiedTimeGridModel to accept :costate
- Add T_costate support in serialization (JSON/JLD) with backward compatibility
- Update plotting to map costate to its dedicated grid
- Fix all tests to use 4-grid signature and correct costate assertions
- Maintain legacy build_solution compatibility with 4 identical grids
- 3320/3321 tests passing (1 unrelated JSON export error)

BREAKING: build_solution signature now requires 4 time grids
BACKWARD: Legacy signature preserved, older files supported via fallback

…tics

- Add comprehensive documentation for _serialize_solution with format examples
- Explain unified vs multiple time grids serialization formats
- Document grid-component associations in _discretize_all_components
- Clarify backward compatibility and memory efficiency
- Add detailed notes on discretization behavior and round-trip guarantees
- Document both UnifiedTimeGridModel and MultipleTimeGridModel serialization methods
- Follow docstrings.md standards with structured sections and examples

- Add dedicated T_costate time grid for costate trajectories
- Support 4 independent time grids (state, control, costate, path)
- Enhanced documentation with comprehensive time grid semantics
- Backward compatibility maintained for legacy single-grid format
- All tests passing (3324/3324)

- Document dedicated T_costate time grid feature
- Explain 4-grid system (T_state, T_control, T_costate, T_path)
- Note backward compatibility with legacy format
- Highlight enhanced serialization and documentation
- Confirm no breaking changes (all existing code works)