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/model/src/lib.rs

@@ -68,8 +68,8 @@ pub use sections::{BeamProfile, Extent, Formulation, Integration, Section, Secti
 pub use sets::Set;
 pub use steps::{
     ArcConstraint, ArcLength, BulkViscosity, DisplacementLimit, ExplicitDynamic, Hourglass, HourglassKind, Imperfection, InitialVelocity,
-    MAX_LOAD_PATH_POINTS, MAX_NONLINEAR_INCREMENTS, MAX_NONLINEAR_ITERATIONS, NonlinearStatic, OutputVariable, Procedure, RigidWall, Step,
-    StressRate, Tolerances, Transient,
+    MAX_LOAD_PATH_POINTS, MAX_NONLINEAR_INCREMENTS, MAX_NONLINEAR_ITERATIONS, MassScaling, NonlinearStatic, OutputVariable, Procedure, RigidWall,
+    Step, StressRate, Tolerances, Transient,
 };
 pub use units::UnitSystem;