# Sprint Contract: MITC4 Material Stiffness Extraction

## Objective
Move MITC4 material matrix, covariant-to-local transform, integration scaffolding, drilling stabilization, stiffness assembly, and internal force helpers into the documented `Element` and `Material` module boundaries.

## Required Reading
- /AGENTS.md
- /PROGRESS.md
- /PLAN.md
- /docs/README.md
- /docs/HARNESS_ENGINEERING.md
- /docs/ARCHITECTURE.md
- /docs/ADR.md
- /docs/NUMERICAL_CONVENTIONS.md
- /docs/MITC4_FORMULATION.md
- /docs/VERIFICATION_PLAN.md
- /phases/1-structure-alignment-refactor/step0-architecture-map.md
- /phases/1-structure-alignment-refactor/step6.md

## Scope
- Extract plane-stress MITC4 material matrix helpers and material diagnostics.
- Extract `2 x 2 x 2` integration data, stiffness accumulation, local/global transform, drilling stabilization, `MITC4ElementKernel`, and internal force helpers.
- Keep `drilling_stiffness_scale = 1.0e-3` and reference-diagonal policy unchanged.
- Keep `Element` responsible for MITC4 element kernel behavior; keep reusable material-law helpers under `Material` if separated.

## Allowed Files
- `include/fesa/fesa.hpp`
- `include/fesa/Element/`
- `src/Element/`
- `include/fesa/Material/`
- `src/Material/`
- `include/fesa/Math/`
- `tests/`
- `CMakeLists.txt`
- `phases/1-structure-alignment-refactor/index.json`
- `PLAN.md`
- `PROGRESS.md`

## Explicit Non-Goals
- Do not change MITC4 stiffness formulas, drilling policy, integration order, or patch-test tolerances.
- Do not add stress/resultant output.
- Do not add reduced integration, S4R, hourglass control, nonlinear tangent stiffness, or pressure loads.
- Do not change assembly behavior beyond include dependency repair.

## Tests To Write First
- Add or update include/link tests for MITC4 stiffness/material module headers.
- Preserve stiffness symmetry, rigid body, drilling sensitivity, internal force, patch, and locking-sensitivity tests as characterization tests.

## Reference Artifacts
- None.

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

## Evaluator Checklist
- MITC4 material and stiffness code no longer lives in the umbrella header as implementation.
- Drilling policy matches `docs/MITC4_FORMULATION.md`.
- Element patch and locking-sensitivity tests still pass.
- No unsupported Abaqus behavior or result output is added.
- Public facade includes remain compatible.

## Handoff Requirements
- Update this step status in `phases/1-structure-alignment-refactor/index.json`.
- Record moved MITC4 material/stiffness files and any remaining MITC4 implementation still left in `fesa.hpp`.

## Do Not
- Do not optimize or parallelize element stiffness in this relocation step.
- Do not use reference displacement mismatch to retune formulation parameters.