# Step 0: property-base-contract

## Read First

Read these files before editing:

- `/AGENTS.md`
- `/docs/PLAN.md`
- `/docs/PROGRESS.md`
- `/docs/WORKNOTE.md`
- `/docs/AGENT_RULES.md`
- `/docs/ARCHITECTURE.md`
- `/docs/ADR.md`
- `/docs/implementation-plans/domain-runtime-storage-implementation-plan.md`
- `/include/fesa/property/ShellProperty.hpp`
- `/include/fesa/core/Domain.hpp`
- `/src/core/Domain.cpp`

## Task

Create `/docs/implementation-plans/property-model-foundation-implementation-plan.md`.

The plan must define the runtime property model contract:

- `Property` is the base class for element properties and sections.
- `PropertyKind` identifies concrete property families; phase 1 supports only `Shell`.
- `ShellProperty` derives from `Property`.
- `Domain` owns property objects through RAII.
- `Domain` validates duplicate property ids and missing material ids.
- `Element::propertyId()` remains an id reference; elements do not own property objects.

Keep out of scope:

- shell section stiffness;
- material constitutive matrices;
- shear correction;
- element formulation;
- assembly, solver, HDF5 output, and reference comparison.

## Tests To Write First

- Documentation-only step. No C++ test is required in this step.

## Acceptance Criteria

```powershell
python -m unittest discover -s scripts -p "test_*.py"
```

## Verification Notes

1. Confirm the plan does not change MITC4 formulation or tolerance policy.
2. Confirm the plan keeps solver state out of `Domain`.
3. Update `phases/property-model-foundation/index.json` for this step result.