Interface FrictionRow

One sparse friction row of [ZeroLengthTable]: the row's one Friction (2D) / Friction3 (3D) — see their doc comments for the field meanings. normal_dof must already have a materials entry on the same row, and the shear DOFs must not (checked by core via debug_assert).

interface FrictionRow {
    b: number;
    k0: number;
    mu: number;
    normalDof: number;
    row: number;
    shearDofs: number[];
}