[Mass] Introduce element-agnostic FEM mass

[alxbilger]

This PR adds a new finite-element mass implementation that builds a sparse global mass matrix from per-element quadrature, using nodal density values instead of a single uniform density.
It also introduces support for computing both kinetic and gravitational potential energy from that mass matrix, along with the corresponding force and matrix-assembly paths.
A new generic nodal-property abstraction was added so density can be stored per node and validated against the mechanical state’s scalar type.
The finite-element helpers were extended with shape-function evaluation and interpolation utilities needed for the mass integration.
The validation example was updated to use the new nodal-density-driven FEM mass setup.

[with-all-tests]

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