Explicit dynamics core: central differences, one-point hex8 with hourglass control, nodal-pressure tet4, solve.explicit (M10 X1) (#27)

* Model: explicit dynamic steps (end time, increment scale and cap, output intervals, initial velocities, rigid walls, hourglass and bulk viscosity controls); deck writers refuse them

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

* fem: uniform-strain hex8 and tet4 kinematics, Flanagan–Belytschko hourglass shape vectors, element frequency bound

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

* Explicit solver crate (L2): central differences with lumped mass, one-point hex8 with Flanagan–Belytschko hourglass control, Bonet–Burton tet4, bulk viscosity, rigid walls, energy account; wave and impact verification

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

* Engine: solve.explicit and solve.history commands, explicit results in the session and solve.probe; cantilever period against the modal solver

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

* Docs: explicit solver in the architecture, roadmap (M10 started), parity and literature (Part H)

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

* Simplify: Model::select_step shared by both solvers, Procedure::sets/is_dynamic, fused per-increment passes without allocation, shared element tail, generic command caps

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

* Catalogue: solve.explicit is ready; solve.history beside solve.probe

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

* Rebase onto the implicit couplings work: Procedure is no longer Copy

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

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/decks/src/abaqus.rs

@@ -216,6 +216,9 @@ fn check_expressible(model: &Model) -> Result<()> {
             ));
         }
     }
+    for step in model.steps().iter().filter(|s| matches!(s.procedure, Procedure::ExplicitDynamic(_))) {
+        bad.insert(format!("step `{}`: explicit dynamic steps (not written to Abaqus input yet)", step.name));
+    }
     for (name, c) in model.couplings() {
         if c.kind == CouplingKind::Distributing && !(1..=3).all(|d| c.dofs.contains(d)) {
             bad.insert(format!("coupling `{name}`: an Abaqus distributing coupling always couples translations 1-3"));
@@ -787,6 +790,9 @@ impl Deck {
                         self.comment("loads have no effect on natural frequencies and are not written");
                     }
                 }
+                Procedure::ExplicitDynamic(_) => {
+                    return Err(Error::Unsupported { format: FORMAT, what: format!("step `{}`: explicit dynamic steps", step.name) });
+                }
             }
             self.output(step);
             self.line("*END STEP");

crates/decks/src/lsdyna/mod.rs

@@ -235,6 +235,9 @@ fn plan<'a>(model: &'a Model, r: &Resolved) -> Result<Plan<'a>> {
     }
     let mut gravity_sets: BTreeSet<Option<&str>> = BTreeSet::new();
     for step in model.steps() {
+        if matches!(step.procedure, Procedure::ExplicitDynamic(_)) {
+            refused.insert(format!("step `{}`: explicit dynamic steps (this writer writes implicit analyses so far)", step.name));
+        }
         let is_static = matches!(step.procedure, Procedure::Static {});
         for b in &step.boundary_conditions {
             let Some(bc) = model.boundary_conditions().get(b) else { continue };
@@ -584,6 +587,9 @@ fn write_step(d: &mut Deck, model: &Model, r: &Resolved, plan: &Plan, step: &Ste
                 d.card([Field::Blank, Field::Blank, Field::Blank, Field::Blank, Field::from(1)]);
             }
         }
+        Procedure::ExplicitDynamic(_) => {
+            return Err(Error::Unsupported { format: FORMAT, what: format!("step `{}`: explicit dynamic steps", step.name) });
+        }
     }
     d.line("*CONTROL_TERMINATION");
     d.header(&["endtim"]);

crates/decks/src/nastran/mod.rs

@@ -234,6 +234,9 @@ struct Deck<'m> {
 
 /// The solution sequence of a list of steps, or why there is none.
 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));
+    }
     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) {
@@ -581,8 +584,8 @@ impl<'m> Deck<'m> {
             comment(&mut out, "eigenvalue extraction");
         }
         for sub in &self.subcases {
-            if let (Some(sid), Procedure::Frequency { modes, min_hz, max_hz }) = (sub.method, sub.step.procedure) {
-                Card::new("EIGRL").int(sid).opt(min_hz).opt(max_hz).int(modes).write(&mut out)?;
+            if let (Some(sid), Procedure::Frequency { modes, min_hz, max_hz }) = (sub.method, &sub.step.procedure) {
+                Card::new("EIGRL").int(sid).opt(*min_hz).opt(*max_hz).int(*modes).write(&mut out)?;
             }
         }
         out.push_str("ENDDATA\n");