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/explicit/src/lib.rs

@@ -46,7 +46,7 @@ pub mod system;
 
 pub use elements::Failure;
 pub use integrate::solve_explicit;
-pub use result::{ContactSummary, ControllingElement, Energies, ErodedElement, ExplicitResult, Frame, History};
+pub use result::{ContactSummary, ControllingElement, Energies, ErodedElement, ExplicitResult, Frame, History, MassScalingSummary};
 
 #[derive(Debug, Clone, PartialEq, thiserror::Error)]
 pub enum Error {