feat: add property model foundation

phases/property-model-foundation/step1.md

@@ -0,0 +1,52 @@
+# Step 1: property-base-interface
+
+## Read First
+
+Read these files before editing:
+
+- `/AGENTS.md`
+- `/docs/AGENT_RULES.md`
+- `/docs/implementation-plans/property-model-foundation-implementation-plan.md`
+- `/include/fesa/core/ModelTypes.hpp`
+- `/include/fesa/property/ShellProperty.hpp`
+- `/tests/property/shell_property_test.cpp`
+- `/CMakeLists.txt`
+
+## Task
+
+Add the runtime property base interface.
+
+Required API:
+
+- File: `/include/fesa/property/Property.hpp`
+- Namespace: `fesa::property`
+- `enum class PropertyKind { Shell };`
+- `class Property`
+  - virtual destructor;
+  - `virtual PropertyId id() const noexcept = 0;`
+  - `virtual PropertyKind kind() const noexcept = 0;`
+
+Rules:
+
+- Use `fesa::core::PropertyId`.
+- Do not add section stiffness or material behavior.
+- Keep this header independent from HDF5, MKL, TBB, and parser headers.
+
+## Tests To Write First
+
+- Add `/tests/property/property_base_test.cpp`.
+- Test that a small derived class can be used through `const Property&`.
+- Test virtual deletion through `std::unique_ptr<Property>`.
+
+## Acceptance Criteria
+
+```powershell
+ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model-object
+python scripts/validate_workspace.py
+```
+
+## Verification Notes
+
+1. Run targeted CTest before implementation and confirm the expected compile failure.
+2. Keep all code C++17/MSVC-compatible.
+3. Update `phases/property-model-foundation/index.json` for this step result.