explicit: hyperelastic materials on the shared finite-strain material point

Neo-Hookean, Mooney–Rivlin and Yeoh materials in explicit dynamics, in
total form from the deformation gradient (objective by construction):
the one-point hex8 takes its mean F over the undeformed element and the
stress τ/J; tet4 takes the isochoric stress of its own F with the
average nodal pressure of the decoupled volumetric energy (Bonet–Burton
at finite strain). Each element's frequency bound follows the tangent's
largest normal stiffness. Small-displacement steps and thermal strain of
hyperelastic elements are refused by name.

Verified by uniaxial strain to λ = 1.5 of all three laws on hex8 and
tet4 against the material point (to rounding), and the oracle case
explicit-neo-hookean-implicit: a quasi-static explicit pull against the
implicit nonlinear solver (lateral contraction to 0.09%, stress to 0.03%).

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

tests/oracle/explicit-neo-hookean-implicit/README.md

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+# Neo-Hookean cube: explicit against implicit (quasi-static)
+
+A unit cube of 2 × 2 × 2 cells of tet4 (six per cell) of a compressible neo-Hookean material
+(E = 10, ν = 0.3, ρ = 1), quarter symmetric (symmetry planes x = 0, y = 0, z = 0), pulled on
+x = 1 to λ = 1.5 in uniaxial stress. The same model is solved by the implicit nonlinear solver
+and by explicit dynamics with the pull applied over 100 time units (a few hundred wave transits):
+the lateral contraction and the axial stress must agree, both coming from the shared
+finite-strain material point.