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/model/src/check.rs

@@ -244,8 +244,14 @@ impl Model {
         let needs_mass = self
             .steps
             .iter()
-            .find(|s| matches!(s.procedure, Procedure::Frequency { .. }))
-            .map(|s| format!("step `{}` extracts frequencies", s.name))
+            .find_map(|s| {
+                let why = match s.procedure {
+                    Procedure::Frequency { .. } => "extracts frequencies",
+                    Procedure::ExplicitDynamic(_) => "is dynamic",
+                    Procedure::Static {} => return None,
+                };
+                Some(format!("step `{}` {why}", s.name))
+            })
             .or_else(|| self.loads.iter().find(|(_, l)| matches!(l, Load::Gravity { .. })).map(|(n, _)| format!("load `{n}` is gravity")));
         if let Some(why) = needs_mass {
             let used: BTreeSet<&str> = self.sections.values().map(|s| s.material.as_str()).collect();
@@ -340,13 +346,18 @@ impl Model {
         }
         for s in &self.steps {
             let subject = format!("step `{}`", s.name);
-            match s.procedure {
+            let prescribes_motion = s
+                .boundary_conditions
+                .iter()
+                .filter_map(|b| self.boundary_conditions.get(b))
+                .any(|b| (1..=6).any(|dof| b.value(dof).is_some_and(|v| v != 0.0)));
+            for set in s.procedure.sets() {
+                if set_of(set).nodes.is_empty() {
+                    d.error(&subject, format!("set `{set}` has no nodes"));
+                }
+            }
+            match &s.procedure {
                 Procedure::Static {} => {
-                    let prescribes_motion = s
-                        .boundary_conditions
-                        .iter()
-                        .filter_map(|b| self.boundary_conditions.get(b))
-                        .any(|b| (1..=6).any(|dof| b.value(dof).is_some_and(|v| v != 0.0)));
                     if s.loads.is_empty() && !prescribes_motion {
                         d.warning(&subject, "nothing loads the model in this step");
                     }
@@ -356,8 +367,15 @@ impl Model {
                         d.warning(&subject, "loads have no effect on natural frequencies");
                     }
                 }
+                Procedure::ExplicitDynamic(e) => {
+                    let moves = e.initial_velocities.iter().any(|v| v.velocity.iter().any(|c| *c != 0.0));
+                    if s.loads.is_empty() && !prescribes_motion && !moves {
+                        d.warning(&subject, "nothing loads or moves the model in this step");
+                    }
+                }
             }
-            if s.boundary_conditions.is_empty() {
+            // A free body is a natural case of transient dynamics (flight, impact).
+            if s.boundary_conditions.is_empty() && !s.procedure.is_dynamic() {
                 d.warning(&subject, "no boundary condition is active, so the model is free to move as a rigid body");
             }
         }