Interface FiberTable

Fibers for every DispBeamColumn/ForceBeamColumn section, flattened and offset-indexed: section k occupies section_offsets[k]..section_offsets[k + 1] in y/z/area/material. section_offsets therefore has num_sections + 1 entries. z is empty in a 2D model and parallel to y in a 3D model (biaxial bending) — torsion is deliberately excluded from the fiber loop in core (FiberSection3's doc comment) and supplied instead as each owning table's own g/j fields.