refactor: store runtime objects in domain

phases/domain-runtime-storage/step0.md

@@ -0,0 +1,50 @@
+# Step 0: runtime-storage-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-model-foundation-implementation-plan.md`
+- `/docs/implementation-plans/analysis-model-objects-implementation-plan.md`
+- `/include/fesa/core/Domain.hpp`
+- `/src/core/Domain.cpp`
+
+## Task
+
+Create `/docs/implementation-plans/domain-runtime-storage-implementation-plan.md`.
+
+The plan must state that `Domain` owns runtime model objects as its canonical storage:
+
+- nodes remain value objects under `fesa::core::Node`;
+- elements are stored as `fesa::element::Element` objects;
+- materials are stored as `fesa::material::Material` objects;
+- shell properties are stored as `fesa::property::ShellProperty` runtime objects;
+- loads are stored as `fesa::load::Load` objects;
+- boundary conditions are stored as `fesa::boundary::BoundaryCondition` objects;
+- steps may remain `LinearStaticStepDefinition` until an analysis-step object is introduced, but step indices must reference runtime load and boundary containers;
+- `core::*Definition` classes may remain as parser/factory-local records, but `Domain` must not persist them.
+
+The plan must also list the migration order and identify the C++ tests that will be rewritten or added.
+
+## 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 introduce parser implementation work.
+2. Confirm the plan does not change MITC4 formulation, reference artifacts, or tolerance policy.
+3. Update `phases/domain-runtime-storage/index.json` for this step result.