docs: redefine phase 1 rebaseline steps

phases/1-linear-static-mitc4-rebaseline/step12.md

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+# Sprint Contract: Assembly Sparse Solver Path
+
+## Objective
+Rebuild or verify full-space assembly, reduced free-DOF solve path, solver adapter boundary, and full-vector reaction recovery using the rebuilt MITC4 kernel.
+
+## Required Reading
+- /AGENTS.md
+- /docs/ARCHITECTURE.md
+- /docs/ADR.md
+- /docs/NUMERICAL_CONVENTIONS.md
+- /docs/MITC4_FORMULATION.md
+
+## Scope
+- Assemble global stiffness and external load vectors.
+- Preserve full-space data for `RF = K_full * U_full - F_full`.
+- Apply constrained DOF elimination via `DofManager`.
+- Use the current deterministic test solver or adapter boundary; keep MKL behind a future adapter.
+
+## Allowed Files
+- `include/`
+- `src/`
+- `tests/`
+- `PLAN.md`
+- `PROGRESS.md`
+
+## Explicit Non-Goals
+- Do not expose MKL or TBB APIs directly in solver core.
+- Do not compare stored Abaqus references yet.
+- Do not compute reactions from reduced equations.
+
+## Tests To Write First
+- Small assembled system tests with known displacement.
+- Full-vector `RF` recovery tests.
+- Constrained/free projection tests through assembly.
+- Singular solve diagnostic tests.
+- Sparse pattern deterministic ordering tests if sparse structure exists.
+
+## Reference Artifacts
+- None.
+
+## Acceptance Commands
+```bash
+python scripts/validate_workspace.py
+```
+
+## Evaluator Checklist
+- Full-space stiffness/load/displacement are available or reconstructable for reactions.
+- Solver adapter boundary is preserved.
+- Singular failures remain actionable.
+
+## Handoff Requirements
+- Record assembly and reaction readiness in `PROGRESS.md`.
+
+## Do Not
+- Do not let elements own global equation ids.