Project → plotly.js figure dicts with no Qt/pyvista/plotly import, so
it is unit-tested in the headless job. Mirrors the PyVista renderer's
geometry (grid, nodes, frames, supports, loads, extrusions, local
axes, labels, deformation) while respecting plotly.js's medium:
pixel-sized markers, None-separated line segments, selection as a
second trace (per-segment line colours are impossible), cone traces
for arrows. Pickable traces carry meta.kind + customdata.