docs: redefine phase 1 rebaseline steps

phases/1-linear-static-mitc4-rebaseline/step4.md

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+# Sprint Contract: Validation Singular Diagnostics
+
+## Objective
+Strengthen domain and analysis-model validation so invalid inputs and singular-prone models fail before or during solve with actionable diagnostics.
+
+## Required Reading
+- /AGENTS.md
+- /docs/ARCHITECTURE.md
+- /docs/NUMERICAL_CONVENTIONS.md
+- /docs/ABAQUS_INPUT_SUBSET.md
+- /docs/VERIFICATION_PLAN.md
+
+## Scope
+- Validate missing nodes, properties, materials, sets, boundary targets, load targets, active elements, and non-positive thickness.
+- Add singular-prone diagnostics for no free DOFs, untouched free DOFs, missing loads where expected, and unconstrained drilling/rotational risks.
+- Keep mesh quality diagnostics out of Phase 1.
+
+## Allowed Files
+- `include/`
+- `src/`
+- `tests/`
+- `PLAN.md`
+- `PROGRESS.md`
+
+## Explicit Non-Goals
+- Do not implement mesh quality metrics such as aspect ratio, skew, or warpage.
+- Do not solve the global system in this step.
+- Do not hide singular checks in element kernels.
+
+## Tests To Write First
+- Missing node/property/material diagnostics.
+- Missing set used by shell section, boundary, or load.
+- No active elements.
+- No free DOFs.
+- Free DOFs untouched by any active element connectivity.
+- Rotational/drilling weak-constraint diagnostic smoke test.
+
+## Reference Artifacts
+- Negative inline test inputs may be used.
+
+## Acceptance Commands
+```bash
+python scripts/validate_workspace.py
+```
+
+## Evaluator Checklist
+- Diagnostics include node id, element id, property id, material id, set name, DOF component, and source line where available.
+- Mesh quality remains explicitly deferred.
+- Validation does not mutate `Domain`.
+
+## Handoff Requirements
+- Record validation coverage in `PROGRESS.md`.
+- Note any remaining singular-diagnostic gaps in `PLAN.md`.
+
+## Do Not
+- Do not classify poor element shape as a Phase 1 mesh quality failure.