# Step 12: Material and Element Property Hierarchy

> 과거 개발 계획이다. 아래 작업·상태·예시는 현재 실행 지시가 아니다.
> 현재 구현과 검증 한계는 [기능 요약](../../docs/cpp-object-oriented-modular-refactoring/README.md)을 따른다.

## 읽어야 할 파일

- `/AGENTS.md`
- `../../docs/solver-core/README.md`
- `../../docs/solver-core/README.md`
- `../../docs/solver-core/README.md`
- `../../docs/solver-core/README.md`
- `../../docs/cpp-object-oriented-modular-refactoring/README.md`
- `/include/fesa/model/model_types.h`
- `/include/fesa/model/domain.h`, `/src/fesa/model/domain.cpp`
- `/tests/unit/model/model_types_test.cpp`, `/tests/unit/model/domain_test.cpp`
- `/src/fesa/CMakeLists.txt`, `/tests/CMakeLists.txt`
- `/phases/cpp-object-oriented-modular-refactoring/index.json`
- `/phases/cpp-object-oriented-modular-refactoring/step12.md`

## 작업

Requirement `R-MODEL-001`의 material/property type system만 구현한다. Domain polymorphic
ownership migration은 Step 13에서 수행한다.

1. `/tests/unit/materials/material_test.cpp`와
   `/tests/unit/properties/element_property_test.cpp`를 먼저 만든다.
2. `C-MODEL-001` tests create each concrete through `std::unique_ptr<Base>`, verify virtual
   destruction, source/internal identity, concrete kind, exact current fields and invalid input
   rejection. Missing base headers cause RED compile failure.
3. Candidate interfaces:

```cpp
enum class MaterialKind { kIsotropicLinearElastic };

class Material {
 public:
  virtual ~Material() = default;
  virtual MaterialKind Kind() const noexcept = 0;
  virtual const SourceEntityId& SourceId() const noexcept = 0;
};

class IsotropicLinearElasticMaterial final : public Material {
 public:
  double YoungsModulus() const noexcept;
  double PoissonsRatio() const noexcept;
};

enum class ElementPropertyKind { kGeneralBeamSection, kShellSection };

class ElementProperty {
 public:
  virtual ~ElementProperty() = default;
  virtual ElementPropertyKind Kind() const noexcept = 0;
  virtual const SourceEntityId& SourceId() const noexcept = 0;
};
```

4. Create these exact production files and register their `.cpp` sources in CMake:
   - `/include/fesa/materials/material.h`
   - `/include/fesa/materials/isotropic_linear_elastic_material.h`
   - `/src/fesa/materials/isotropic_linear_elastic_material.cpp`
   - `/include/fesa/properties/element_property.h`
   - `/include/fesa/properties/general_beam_section.h`
   - `/include/fesa/properties/shell_section.h`
   - `/src/fesa/properties/general_beam_section.cpp`
   - `/src/fesa/properties/shell_section.cpp`
5. Add concrete `GeneralBeamSection` and `ShellSection` classes preserving their current validated
   fields, units, source identity and optional frame meaning.
6. Move concrete record responsibility out of unrelated `model_types.h` only as required to avoid
   duplicate definitions; update current compile consumers minimally.
7. No base contains density, plastic state, anisotropic tensor, thickness, area or no-op virtual
   methods that are not common to every concrete.

## Acceptance Criteria

과거 도구 실행·출력 지시는 정리했으며, 현재 검증 방법은 기능 요약을 따른다.

## 검증 및 상태 갱신

- RED missing interfaces, concrete validation, virtual ownership and full CTest를 summary에 기록한다.
- Any future-only field/interface or current property regression is an `error`.
- 성공 시 현재 Step만 `completed`로 갱신한다.
- timestamp, retry, commit, advancement는 Executor 소유다.

## 금지사항

- Density/plasticity/anisotropy APIs를 추가하지 마라. 이유: 승인된 현재 behavior가 아니다.
- Domain을 `shared_ptr` repository로 바꾸지 마라.
- ElementFactory 또는 numerical kernel을 수정하지 마라. 이유: Step 14 소유다.
- 직접 commit하거나 hook script를 수동 실행하지 마라.