# Step 6: model-beam-topology

## Read These Files First

Read the following files before editing:

- `/AGENTS.md`
- `/docs/ARCHITECTURE.md`
- `/docs/ADR.md`
- `/docs/implementation-plans/euler-beam-3d-implementation-plan.md`
- `/src/fesa/model/element.hpp`
- `/src/fesa/model/element.cpp`
- `/tests/unit/model_element_test.cpp`

## Task

Use TDD to add a semantic model topology for a two-node 3D beam.

Required production behavior:

- Add `beam2` to `fesa::model::ElementTopology`.
- Preserve existing `truss2`, `bar2`, and `unknown` behavior.
- Do not store equation IDs on `Element`.
- Do not add section constants to `Element` in this step.

## Tests To Write First

Modify `/tests/unit/model_element_test.cpp` first.

Add a failing assertion that constructs an element with:

- id `ElementId{10}`
- topology `ElementTopology::beam2`
- node ids `{NodeId{1}, NodeId{2}}`
- property id `PropertyId{7}`

The test must verify:

- `element.topology() == ElementTopology::beam2`
- `element.node_ids().size() == 2`
- existing `bar2` behavior still works or the test still covers it

RED command:

```powershell
ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model_element_test
```

Expected RED: compile failure because `ElementTopology::beam2` is not defined.

Then implement the minimal enum addition.

GREEN command:

```powershell
ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model_element_test
```

## Acceptance Criteria

```powershell
ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model_element_test
python -m unittest discover -s scripts -p "test_*.py"
python scripts/validate_workspace.py
```

## Verification Notes

1. Confirm the test failed before editing production code.
2. Confirm no unrelated model refactor was made.
3. Update `phases/euler-beam-3d/index.json` step 6:
   - success: `"status": "completed"`, `"summary": "beam2 model topology added with unit test"`
   - failure after retries: `"status": "error"`, `"error_message": "<specific error>"`
   - blocked: `"status": "blocked"`, `"blocked_reason": "<specific reason>"`

## Forbidden

- Do not create parser code.
- Do not add beam stiffness code in this step.