# Sprint Contract: MITC4 Covariant Strain Tying

## Objective
Implement or rebuild the degenerated-continuum displacement interpolation, direct covariant strain rows, and MITC transverse shear tying interpolation.

## Required Reading
- /AGENTS.md
- /docs/MITC4_FORMULATION.md
- /docs/NUMERICAL_CONVENTIONS.md

## Scope
- Implement five-DOF local displacement interpolation.
- Transform global rotations to local `alpha`, `beta`, and `gamma`.
- Compute direct covariant strain rows in documented internal strain order.
- Replace `eps_13` and `eps_23` with midside MITC tying interpolation.
- Add finite-difference tests for strain rows.

## Allowed Files
- `include/`
- `src/`
- `tests/`
- `PLAN.md`
- `PROGRESS.md`

## Explicit Non-Goals
- Do not integrate stiffness.
- Do not add material transform.
- Do not compute MITC shear from Cartesian `gamma_xz` and `gamma_yz` first.

## Tests To Write First
- Local rotation transform tests for `alpha`, `beta`, `gamma`.
- Displacement interpolation tests at midsurface and through-thickness points.
- Direct covariant strain finite-difference tests.
- Tying-row tests at `A`, `B`, `C`, and `D`.
- Gauss-point MITC shear interpolation tests proving rows come from tying rows.

## Reference Artifacts
- None.

## Acceptance Commands
```bash
python scripts/validate_workspace.py
```

## Evaluator Checklist
- Strain vector ordering is documented or explicitly permuted.
- `gamma` does not contribute to physical continuum strain.
- FESA sign convention for `A/C/B/D` is tested.

## Handoff Requirements
- Record strain/tying readiness in `PROGRESS.md`.

## Do Not
- Do not reintroduce direct Gauss-point transverse shear for MITC rows.