feat: rebuild mitc4 stiffness drilling

PROGRESS.md

@@ -13,10 +13,37 @@ Every new agent session must read this file together with `PLAN.md` before plann
 - Do not remove history unless the user explicitly asks for archival cleanup.
 
 ## Current Status
-Phase 1 has a new rebaseline phase definition in `phases/1-linear-static-mitc4-rebaseline`. Steps 0 through 9 are complete. `quad_02_phase1.inp` is now the normalized Phase 1-compatible input path for the stored `quad_02` S4 reference pair, while the original `quad_02.inp` remains preserved unsupported provenance. Core numeric aliases, DOF mapping, validation harness, model diagnostic context, the Phase 1 parser/domain subset, validation/singular diagnostics, DofManager/reaction foundation, minimum result model metadata, displacement CSV comparator foundation, MITC4 geometry/director scaffolding, MITC4 displacement/strain/tying row scaffolding, and MITC4 material/transform/integration scaffolding have been revalidated. The old `phases/1-linear-static-mitc4` path is historical and superseded after the MITC4 formulation reset.
+Phase 1 has a new rebaseline phase definition in `phases/1-linear-static-mitc4-rebaseline`. Steps 0 through 10 are complete. `quad_02_phase1.inp` is now the normalized Phase 1-compatible input path for the stored `quad_02` S4 reference pair, while the original `quad_02.inp` remains preserved unsupported provenance. Core numeric aliases, DOF mapping, validation harness, model diagnostic context, the Phase 1 parser/domain subset, validation/singular diagnostics, DofManager/reaction foundation, minimum result model metadata, displacement CSV comparator foundation, MITC4 geometry/director scaffolding, MITC4 displacement/strain/tying row scaffolding, MITC4 material/transform/integration scaffolding, and MITC4 stiffness/drilling/internal-force scaffolding have been revalidated. The old `phases/1-linear-static-mitc4` path is historical and superseded after the MITC4 formulation reset.
 
 ## Completed Work
 
+### 2026-05-04 - P1R-10 MITC4 stiffness and drilling completed
+Author: Codex
+
+Changed files:
+- `include/fesa/fesa.hpp`
+- `tests/test_main.cpp`
+- `phases/1-linear-static-mitc4-rebaseline/index.json`
+- `PLAN.md`
+- `PROGRESS.md`
+
+Summary:
+- Added tests and implementation for MITC4 element stiffness accumulation using tied strain rows, transformed material matrices, and `2 x 2 x 2` integration samples.
+- Added a stiffness result API that exposes local physical stiffness, local drilling-stabilized stiffness, global stiffness, integration-point count, drilling reference diagonal, and drilling stiffness metadata.
+- Replaced the old default drilling scale with the documented `drilling_stiffness_scale = 1.0e-3` and applied it to the minimum positive physical local stiffness diagonal before global transformation.
+- Added diagnostics for invalid drilling scale and missing positive drilling reference diagonal.
+- Added tests for stiffness symmetry, drilling scale sensitivity, rigid translation zero energy, documented drilling-only energy, and `f_int_e = K_e u_e` consistency.
+- Routed `MITC4ElementKernel::stiffness` through the rebuilt formulation path so later assembly uses the Step 7 through Step 10 MITC4 helpers.
+
+Verification:
+- First ran `python scripts/validate_workspace.py` after adding Step 10 tests; it failed as expected because the MITC4 stiffness/drilling/internal-force APIs did not exist yet.
+- After implementation, `python scripts/validate_workspace.py` configured CMake, built `fesa_core` and `fesa_tests`, and ran CTest successfully.
+- CTest result: 1 test executable passed.
+
+Follow-up:
+- Continue with P1R-11 MITC4 patch, locking-sensitivity, and benchmark tests.
+- Step 10 intentionally did not tune the drilling scale to a reference case, add stress/resultant output, introduce reduced integration/hourglass logic, or rebuild sparse/global assembly beyond routing the existing kernel to the new element formulation.
+
 ### 2026-05-04 - P1R-09 MITC4 material integration scaffolding completed
 Author: Codex