modify documents

docs/MITC4_FORMULATION.md

@@ -164,6 +164,17 @@ Before integration with the global solver:
 - bending-dominated sanity case.
 - drilling stiffness sensitivity check.
 
+## Pre-Implementation Gate
+Do not implement `MITC4Element` until these decisions are recorded in this document or a linked ADR:
+- exact transverse shear tying point equations.
+- exact local shell basis algorithm for flat and non-flat quadrilaterals.
+- exact default drilling stiffness scale and parameter name.
+- exact integration point ordering.
+- whether Phase 1 reports only `U` and `RF`, or also element stress/strain/resultant fields.
+- stress/strain recovery locations if `S`, `E`, or `SF` are included in Phase 1.
+
+If implementation starts before all optional stress/resultant decisions are closed, limit Phase 1 mandatory outputs to `U` and `RF`.
+
 ## Reference Benchmarks
 MITC4 baseline acceptance should include:
 - single-element membrane test.
@@ -176,6 +187,15 @@ MITC4 baseline acceptance should include:
 
 Distorted mesh tests should be added after the baseline passes, but Phase 1 does not implement general mesh quality diagnostics.
 
+Current stored reference artifact:
+- `references/quad_01.inp`
+- `references/quad_01_displacements.csv`
+
+Compatibility note:
+- `quad_01.inp` is an Abaqus-generated S4R/NLGEOM reference input and is not a Phase 1 MITC4 parser acceptance case.
+- It should be used as an external reference artifact and future compatibility target unless a normalized Phase 1 `S4` input is added.
+- The displacement CSV can still exercise `U` field comparison infrastructure once node ids and component mapping are supported.
+
 ## Future Extensions
 Geometric nonlinearity:
 - Add updated geometry, current frame handling, tangent stiffness, and Newton-Raphson integration.