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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/fem.md

@@ -468,3 +468,15 @@ geometry and take no thermal strain yet (both refused by name). Verified by unia
 λ = 1.5 of all three laws on hex8 and tet4 against the material point (`crates/explicit/tests/hyperelastic.rs`)
 and by `tests/oracle/explicit-neo-hookean-implicit`, a quasi-static explicit pull against the
 implicit nonlinear solver.
+
+## Mass scaling (explicit dynamics)
+
+Opt-in per explicit step (`mass_scaling: {target_increment, set}`): every solid element (of the
+set, or of the model) whose stable increment times the step's scale is below the target has its
+density raised by (target / increment)², which brings its increment to the target (its lumped
+masses, frequency bound and hourglass and bulk-viscosity scales follow; adiabatic heating keeps
+the physical density). The run reports the elements scaled, the added mass in all and per
+section, its fraction of the model's mass, a warning, and the kinetic energy of the added mass
+(`energy.added_kinetic`, part of the kinetic energy, so the balance holds). Verified on a uniform
+bar scaled to twice its increment: added mass three times the bar's, the increment doubled, the
+added mass's share of the kinetic energy three quarters (`crates/explicit/tests/one_d.rs`).

docs/parity.md

@@ -64,7 +64,7 @@ products are run, no documentation text is copied). Status: **done**, **started*
 | Decks | Readers keeping unknown keywords | M9 | | planned |
 | Solve | Own linear static and modal solver, verified against analytic solutions | M4 | yes | started (solid elements: tet4, tet10, wedge6, hex8, hex20; 2D plane stress, plane strain and axisymmetric tri3, tri6, quad4 (full, incompatible modes), quad8 (full, reduced); quad4/tri3 MITC shells and Timoshenko beams with profiles, mixed with solids; reference points, kinematic and distributing couplings, moments; centrifugal loads) |
 | Solve | Own nonlinear implicit solver: Newton–Raphson with line search and automatic increments, total Lagrangian solids and 2D elements, mixed u–p at finite strain, follower pressure | M15 | yes | started (solids, plane strain, axisymmetric; St Venant–Kirchhoff, neo-Hookean and J2 plasticity (Simo's exponential return map at finite strain, radial return at small strain, consistent tangents, the return shared with explicit dynamics); verified by the elastica, tube inflation, patch and objectivity tests, uniaxial tension to large strain, Hill's thick cylinder, a cantilever's collapse and the Bauschinger effect; Mooney–Rivlin and Yeoh; thermal strain at finite and small strain; a symmetric indefinite LDLᵀ factorization with inertia, Newton through indefinite tangents (hybrid elements at ν = 0.4999 under prescribed displacement); linear buckling of solids (Euler columns, Bryan's plate) and imperfections from its modes; arc-length with critical points from the inertia (shallow-arch snap-through); shells, implicit contact next) |
-| Solve | Own explicit dynamics solver (central differences, one-point hex8 with hourglass control, nodal-pressure tet4, bulk viscosity, energy balance), verified against analytic wave and impact solutions | M10 | yes | started (X1 core and X2: large deformation, objective rates, J2 and Johnson–Cook plasticity, damage, erosion; X3: deformable contact and friction, verified by bar collision, sliding blocks, the contact patch test and Hertz) |
+| Solve | Own explicit dynamics solver (central differences, one-point hex8 with hourglass control, nodal-pressure tet4, bulk viscosity, energy balance), verified against analytic wave and impact solutions | M10 | yes | started (X1 core and X2: large deformation, objective rates, J2 and Johnson–Cook plasticity, damage, erosion; X3: deformable contact and friction, verified by bar collision, sliding blocks, the contact patch test and Hertz; hyperelastic materials on the shared material point; opt-in mass scaling) |
 | Results | Deformed shape, contours, mode shapes, probes, reaction sums | M4 | | started (results in the session, `solve.probe`, reaction sums; display planned) |
 | Results | XY and path plots, cuts, display groups, free-body forces, animation | M9 | | planned |
 | Results | Readers for public result formats (Nastran OP2, LS-DYNA d3plot, Abaqus `.fil`) | M9 | | planned |