model: nonlinear static steps and neo-Hookean hyperelasticity, mapped by the deck writers

A nonlinear_static procedure (nonlinear geometry, initial/min/max increments,
increment and iteration caps, force/displacement/energy tolerances, line
search), validated with caps against hostile values. Materials gain an
explicit hyperelastic option (decoupled compressible neo-Hookean on the
isotropic constants). Abaqus writes *STEP, NLGEOM, INC / *STATIC with the
increments and *HYPERELASTIC, NEO HOOKE; LS-DYNA writes the ramped implicit
run with full Newton and *CONTROL_IMPLICIT_AUTO; Nastran refuses SOL 400 by
name, as LS-DYNA does hyperelastic materials in nonlinear steps. Explicit
dynamics refuses hyperelastic materials by name; solve.run names the solver
still to come.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/decks/src/nastran/mod.rs

@@ -280,6 +280,9 @@ fn solution(steps: &[&Step]) -> std::result::Result<u32, String> {
     if let Some(s) = steps.iter().find(|s| matches!(s.procedure, Procedure::ExplicitDynamic(_))) {
         return Err(format!("step `{}`: explicit dynamic steps (Nastran's linear solutions have none)", s.name));
     }
+    if let Some(s) = steps.iter().find(|s| matches!(s.procedure, Procedure::NonlinearStatic(_))) {
+        return Err(format!("step `{}`: nonlinear static steps (Nastran's SOL 400; this writer writes SOL 101 and 103 so far)", s.name));
+    }
     let statics = steps.iter().find(|s| matches!(s.procedure, Procedure::Static {}));
     let modes = steps.iter().find(|s| matches!(s.procedure, Procedure::Frequency { .. }));
     match (statics, modes) {