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/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(