# Sprint Contract: MITC4 Stiffness Drilling

## Objective
Assemble the MITC4 element stiffness and internal force kernel using the documented strain, material, integration, six-DOF transform, and drilling stabilization.

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

## Scope
- Accumulate `K_e = integral B^T D B dV`.
- Add local drilling stabilization with `drilling_stiffness_scale = 1.0e-3`.
- Transform local element stiffness/internal force to global 24-DOF order.
- Preserve parameterization and diagnostics for drilling scale.

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

## Explicit Non-Goals
- Do not add assembly/global solve.
- Do not tune drilling scale to match one reference case.
- Do not output stresses/resultants as mandatory fields.

## Tests To Write First
- Stiffness dimension and symmetry tests.
- Drilling diagonal reference tests using minimum positive physical local stiffness diagonal.
- Zero/invalid reference diagonal diagnostic tests.
- Rigid body physical strain-energy tests with documented drilling effects.
- Internal force consistency test `f_int_e = K_e u_e` for linear elastic Phase 1.

## Reference Artifacts
- None.

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

## Evaluator Checklist
- Old `1.0e-6 * E * thickness` drilling rule is gone.
- Global 24-DOF ordering matches `UX,UY,UZ,RX,RY,RZ` per node.
- Stiffness remains linear elastic and symmetric.

## Handoff Requirements
- Record element kernel readiness in `PROGRESS.md`.

## Do Not
- Do not optimize or parallelize before element tests pass.