1D elements I: trusses (rod2, rod3), point masses, rotary inertia, springs and dashpots (#38)

* Model, fem and decks: point masses, rotary inertia, springs, dashpots and rod3 trusses

Model: PointMass and Spring definitions (spring | dashpot; to ground or
two-point; along a degree of freedom, a vector or the line of the ends) on a
set or reference point, resolved per node; ElementKind::Rod3. fem: rod2/rod3
truss kernel on Basis::Line2/Line3 declaring ElementDofs (translations).
Decks: Abaqus T3D3, MASS, ROTARYI, SPRING1/2/A, DASHPOT1/2/A; Nastran CONM1,
CELAS2, CDAMP2; LS-DYNA ELEMENT_MASS/INERTIA and ELEMENT_DISCRETE; what a
format cannot take is reported. Commands model.mass.* and model.spring.*.

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

* Implicit solver: trusses, point masses, rotary inertia and springs on the DofLayout

Trusses declare their translations like solids; point masses, inertia and
springs ask the layout for the kinds they need at their nodes (translations,
rotations), and reference points they use get six degrees of freedom. The
matrix pattern takes rows for those points; dashpots are ignored with a
warning. Existing results are bit-identical (hash-checked).

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

* Explicit solver: trusses, point masses, springs and dashpots on the node-major layout

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

* docs: fem.md on trusses and discrete elements in the layout; regenerate features.md

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

* docs: trusses in plane models and corotational explicit trusses

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/engine/src/cmd/define.rs

@@ -1,13 +1,14 @@
 //! Model definition commands: create, edit and delete sets, materials, sections, reference
-//! points, couplings, boundary conditions, loads, fluid domains and steps, and the step activity
-//! table.
+//! points, couplings, point masses, springs and dashpots, boundary conditions, loads, fluid
+//! domains and steps, and the step activity table.
 //!
 //! Each definition's parameters are its JSON form in the model (the same form a `.modelcraft`
 //! file stores), plus its `name`. `create` fails when the name is taken, `edit` replaces an
 //! existing definition, `delete` refuses while another definition refers to it.
 
 use modelcraft_model::{
-    BoundaryCondition, Coupling, FaceCondition, FluidBoundary, FluidDomain, Load, Material, Model, ReferencePoint, Section, Set, Step,
+    BoundaryCondition, Coupling, FaceCondition, FluidBoundary, FluidDomain, Load, Material, Model, PointMass, ReferencePoint, Section, Set, Spring,
+    Step,
 };
 use serde::de::DeserializeOwned;
 use serde_json::{Value, json};
@@ -176,6 +177,34 @@ definition_commands!(
     COUPLING_DELETE
 );
 
+definition_commands!(
+    "point mass",
+    PointMass,
+    "mass",
+    "Point Mass",
+    r#"{"name": "motor", "target": {"set": "…"} | {"reference_point": "…"}, "mass": 2.5 | [mx, my, mz] (per global direction), "inertia": [I11, I22, I33, I12, I13, I23] (tensor about the node in global axes; needs rotations: a reference point)} — at every node of the target, each with the full value"#,
+    add_mass,
+    replace_mass,
+    remove_mass,
+    MASS_CREATE,
+    MASS_EDIT,
+    MASS_DELETE
+);
+
+definition_commands!(
+    "spring",
+    Spring,
+    "spring",
+    "Spring or Dashpot",
+    r#"{"name": "mount", "type": "spring", "stiffness": 1000 | "type": "dashpot", "coefficient": 5, "first": {"set": "…"} | {"reference_point": "…"}, "second": (optional; without it each node of `first` connects to the ground; with it each end is one node), "direction": {"dof": 1..6} (4–6: torsional) | {"vector": [x, y, z]} | "line" (along the two ends, two-point only)} — dashpots act in dynamic steps; static and frequency steps ignore them"#,
+    add_spring,
+    replace_spring,
+    remove_spring,
+    SPRING_CREATE,
+    SPRING_EDIT,
+    SPRING_DELETE
+);
+
 definition_commands!(
     "boundary condition",
     BoundaryCondition,

crates/engine/src/cmd/mod.rs

@@ -54,6 +54,12 @@ pub static ALL: &[CommandSpec] = &[
     define::COUPLING_CREATE,
     define::COUPLING_EDIT,
     define::COUPLING_DELETE,
+    define::MASS_CREATE,
+    define::MASS_EDIT,
+    define::MASS_DELETE,
+    define::SPRING_CREATE,
+    define::SPRING_EDIT,
+    define::SPRING_DELETE,
     define::BOUNDARY_CONDITION_CREATE,
     define::BOUNDARY_CONDITION_EDIT,
     define::BOUNDARY_CONDITION_DELETE,

crates/engine/src/cmd/solve.rs

@@ -950,4 +950,32 @@ pub(crate) mod tests {
         // Defining steps edited the model, so the impact result is stale.
         assert_eq!(s.solved_steps().count(), 0);
     }
+
+    /// A mass on a spring and a dashpot, defined, solved and written through commands; editing
+    /// and deleting them goes through the same checks as every definition.
+    #[test]
+    fn spring_mass_through_commands() {
+        let mut s = Session::default();
+        let script = json!([
+            {"command": "mesh.node.add", "params": {"label": 1, "xyz": [0, 0, 0]}},
+            {"command": "model.set.create", "params": {"name": "n", "nodes": [1]}},
+            {"command": "model.mass.create", "params": {"name": "m", "target": {"set": "n"}, "mass": 4}},
+            {"command": "model.spring.create", "params": {"name": "k", "type": "spring", "first": {"set": "n"}, "direction": {"dof": 1}, "stiffness": 400}},
+            {"command": "model.spring.create", "params": {"name": "c", "type": "dashpot", "first": {"set": "n"}, "direction": {"vector": [1, 0, 0]}, "coefficient": 1}},
+            {"command": "model.boundary_condition.create", "params": {"name": "yz", "type": "displacement", "target": {"set": "n"}, "values": [null, 0, 0, null, null, null]}},
+            {"command": "model.step.create", "params": {"name": "Modes", "procedure": {"type": "frequency", "modes": 1}, "boundary_conditions": ["yz"]}},
+        ]);
+        s.run_script(&script).unwrap();
+        let r = s.execute("solve.frequency", &json!({})).unwrap();
+        let f = r["modes"][0]["frequency"].as_f64().unwrap();
+        assert!((f - 10.0 / (2.0 * std::f64::consts::PI)).abs() < 1e-12, "{r}");
+        assert!(r["warnings"].as_array().unwrap().iter().any(|w| w.as_str().unwrap().contains("dashpots `c`")), "{r}");
+        assert!(s.execute("model.mass.edit", &json!({"name": "m", "target": {"set": "n"}, "mass": -1})).is_err());
+        assert!(s.execute("model.set.delete", &json!({"name": "n"})).is_err(), "the set is in use");
+        s.execute("model.mass.edit", &json!({"name": "m", "target": {"set": "n"}, "mass": 1})).unwrap();
+        let r = s.execute("solve.frequency", &json!({})).unwrap();
+        assert!((r["modes"][0]["frequency"].as_f64().unwrap() - 20.0 / (2.0 * std::f64::consts::PI)).abs() < 1e-12);
+        s.execute("model.spring.delete", &json!({"name": "c"})).unwrap();
+        assert!(s.model.springs().len() == 1 && s.model.mass("m").is_some());
+    }
 }