[transfix]

The FEM demos' named colormaps become a transfer function the tf_editor widget (#827) edits, as asked once #827 and #820 landed: "for the FEM demos, there are parts where colormaps apply. we should use our new colortable ariadne widget ... where they apply".

What changes

• mesh_lab and fe_lab colour what they show through <node>.field_colors, a cvc::tf::model string: mesh_lab's colour field (curvature, geodesic distance, eigenmodes) and fe_lab's field on the boundary and the cut, and its elements' size or quality. Only the colour ramp is used.
• The colormap keys keep working. color.colormap / fe.view.colormap seed the model and reset it to that colormap's ramp whenever they change. So fem_gallery.ari, which sets a colormap per item, behaves as before, and an edit in the editor stands until the next change.
• An explicit domain in the model is the colour range (in fe_lab it wins over view.fixed_range). mesh_lab's color.min / color.max report the range used.
• The editor's axis and histogram: the nodes publish field_colors.range (the colours' span under an auto domain) and a 128-bin field_colors.histogram. fe_lab's is the whole solution, or the shown elements' size or quality in the elements view.
• mesh_tools and fe_controls each embed a tf_editor ("Colours") under the Colormap combo, with the opacity curve and isocontours hidden.
• GeometryNode::setColorTable: a baked colour table the scalar field maps through instead of the colormap. It is computed on the CPU like the colormaps, so WebGL draws it the same, and it recolours in place.

Tests

• cvcgl_scalar_field: the table's colours, recoloured in place, kept across setColorMap / setScalarField, and cleared by an empty or too-short table.
• cvcgl_ariadne_realize: 8 new checks over both node types: the ramp, its published range and histogram, the colormap reset, and an explicit domain as the colour range.
• Locally on Windows (MSVC Release, before the rebase onto nav: every length and force in the state tree is in world units #824/test(physics): build physics_dyn_bike_plant_test without FP contraction #837): full build clean; 868/868 Ariadne/cvcGL/tf/LBIE/FEM tests; arigl check of fem_gallery.ari and mesh_lab.ari clean. The cvcgl_* tests are not part of CI's recipe jobs.

Docs: ARIADNE.md ("Field colours"), TRANSFER_FUNCTIONS.md, mesh_nodes.h, and the two components' headers.