explicit: opt-in mass scaling to a target increment, the added mass reported

An explicit step may ask for mass scaling to a target time increment (on
a set, or the whole model): elements whose stable increment is below it
get the density that reaches it, with their lumped masses, frequency
bound and hourglass scales; adiabatic heating keeps the physical
density. The run reports the elements scaled, the added mass in all and
per section with its fraction, a warning, and the added mass's kinetic
energy in the energy account. Verified on a uniform bar scaled to twice
its increment (added mass 3×, increment doubled, a quarter of the
kinetic energy left to the physical mass). ROADMAP and parity note the
hyperelastic materials and mass scaling.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/define.rs

@@ -292,7 +292,7 @@ fn fluid_boundary(s: &mut Session, p: &Value) -> Result<Value> {
 pub const STEP_CREATE: CommandSpec = CommandSpec {
     id: "model.step.create",
     label: "New Step",
-    params: r#"{"name": "Bend", "procedure": {"type": "static"} | {"type": "nonlinear_static", "nonlinear_geometry": true, "initial_increment": 0.1, "min_increment": 1e-5, "max_increment": 1.0 (fractions of the step), "max_increments": 100, "max_iterations": 16 (per increment), "tolerances": {"force": 1e-6, "displacement": 1e-5, "energy": 1e-10}, "line_search": false, "load_path": [[0, 0], [0.5, 1], [1, 0]] (optional: the load factor against the step fraction, piecewise linear from [0, 0] to fraction 1; increments end at its points)} (loads and prescribed displacements are the load factor times their values; it ramps from 0 to 1 without a load path) | {"type": "frequency", "modes": 10, "min_hz": 0, "max_hz": 500} | {"type": "fluid", "domain": "Flow", "end_time": 1.0 | "steps": 1000, "time_step": (default: chosen for Mach 0.087 and τ ≤ 1), "frames": 10, "magic": 0.1875 (the two-relaxation-time Λ)} | {"type": "explicit_dynamic", "end_time": 0.001, "dt_scale": 0.9, "max_increments": 10000000, "output_interval", "history_interval", "history_nodes": "set", "initial_velocities": [{"set", "velocity": [vx, vy, vz]}], "rigid_walls": [{"set", "point", "normal", "friction": {"static": 0.3, "kinetic": 0.2, "decay": 10} (Coulomb; default frictionless)}], "hourglass": {"type": "viscous|stiffness", "coefficient": 0.1}, "bulk_viscosity": {"linear": 0.06, "quadratic": 1.5}}, "output": ["displacement", "reaction_force", "stress", "strain"] (default by procedure), "boundary_conditions": [names], "loads": [names] (active in this step; nothing carries over between steps), "before": "step" (default: last)}"#,
+    params: r#"{"name": "Bend", "procedure": {"type": "static"} | {"type": "nonlinear_static", "nonlinear_geometry": true, "initial_increment": 0.1, "min_increment": 1e-5, "max_increment": 1.0 (fractions of the step), "max_increments": 100, "max_iterations": 16 (per increment), "tolerances": {"force": 1e-6, "displacement": 1e-5, "energy": 1e-10}, "line_search": false, "load_path": [[0, 0], [0.5, 1], [1, 0]] (optional: the load factor against the step fraction, piecewise linear from [0, 0] to fraction 1; increments end at its points)} (loads and prescribed displacements are the load factor times their values; it ramps from 0 to 1 without a load path) | {"type": "frequency", "modes": 10, "min_hz": 0, "max_hz": 500} | {"type": "fluid", "domain": "Flow", "end_time": 1.0 | "steps": 1000, "time_step": (default: chosen for Mach 0.087 and τ ≤ 1), "frames": 10, "magic": 0.1875 (the two-relaxation-time Λ)} | {"type": "explicit_dynamic", "end_time": 0.001, "dt_scale": 0.9, "max_increments": 10000000, "output_interval", "history_interval", "history_nodes": "set", "initial_velocities": [{"set", "velocity": [vx, vy, vz]}], "rigid_walls": [{"set", "point", "normal", "friction": {"static": 0.3, "kinetic": 0.2, "decay": 10} (Coulomb; default frictionless)}], "hourglass": {"type": "viscous|stiffness", "coefficient": 0.1}, "bulk_viscosity": {"linear": 0.06, "quadratic": 1.5}, "mass_scaling": {"target_increment": 1e-6, "set": "…" (optional, default every element)} (opt-in: elements below the target increment get the density that reaches it; the added mass is reported)}, "output": ["displacement", "reaction_force", "stress", "strain"] (default by procedure), "boundary_conditions": [names], "loads": [names] (active in this step; nothing carries over between steps), "before": "step" (default: last)}"#,
     form: Some(super::forms::STEP_CREATE),
     kind: CommandKind::Model,
     run: step_create,

crates/engine/src/cmd/forms.rs

@@ -505,6 +505,11 @@ const PROCEDURE: Field = variants(
                 group("bulk_viscosity", "Bulk viscosity", &[positive("linear", "Linear", D::None), positive("quadratic", "Quadratic", D::None)]),
                 flag("nonlinear_geometry", "Nonlinear geometry").def("true"),
                 choice("stress_rate", "Stress rate", &[("jaumann", "Jaumann"), ("green_naghdi", "Green–Naghdi")]),
+                group(
+                    "mass_scaling",
+                    "Mass scaling",
+                    &[positive("target_increment", "Target increment", D::Time).req(), reference("set", "Set", C::Set)],
+                ),
             ],
         ),
         variant(

crates/engine/src/cmd/solve.rs

@@ -355,6 +355,7 @@ fn explicit_summary(r: &ExplicitResult) -> Value {
         "history_samples": r.history.time.len(),
         "eroded": r.eroded,
         "contacts": r.contacts,
+        "mass_scaling": r.mass_scaling,
         "warnings": r.warnings,
     })
 }