Colours that were hard-coded in the renderers now come from the shared
style: nodes, supports, nodal/element loads, section extrusions (+
opacity) and label font size. The PyVista frame LUT becomes a four-slot
palette [beam, truss, link, selected] with the cell scalar carrying the
family slot, so VTK matches the Plotly backend's per-family colouring
that landed earlier. Adds style helpers shared by both backends
(element_family_index, family_palette) and an immutable with_updates().
The style also gains the editable field table (STYLE_FIELDS) the Plot
Properties dialog is built from.
