feat: route both canvas backends through RenderStyle

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.

tests/unit/test_render_style.py

@@ -0,0 +1,117 @@
+"""Unit tests for the shared render style and its editable subset."""
+
+from __future__ import annotations
+
+import dataclasses
+
+import pytest
+
+from otko.core import ElasticBeamColumn, Node, Project, TrussElement, ZeroLengthElement
+from otko.views.canvas3d.style import (
+    EDITABLE_FIELDS,
+    SELECTED_STATE,
+    STYLE_FIELDS,
+    RenderStyle,
+    element_family_index,
+    family_palette,
+)
+
+
+def test_defaults_are_the_documented_palette() -> None:
+    style = RenderStyle()
+    assert style.node_color == "#d9d9d9"
+    assert style.selected_color == "#00ffff"
+    # opstool-derived element palette.
+    assert style.element_beam_color == "#0652ff"
+    assert style.element_truss_color == "#FF8C00"
+    assert style.element_link_color == "#39FF14"
+    assert len(style.response_scale_colors) == 10
+    assert style.response_scale_colors[0] == "#313695"  # cold end
+    assert style.response_scale_colors[-1] == "#a50026"  # warm end
+
+
+def test_with_updates_is_immutable() -> None:
+    original = RenderStyle()
+    updated = original.with_updates(node_color="#123456", label_font_size=20)
+    assert updated is not original
+    assert updated.node_color == "#123456"
+    assert updated.label_font_size == 20
+    # The source instance is untouched (frozen dataclass contract).
+    assert original.node_color == "#d9d9d9"
+    assert original.label_font_size == 12
+
+
+def test_editable_subset_matches_the_field_table() -> None:
+    style = RenderStyle()
+    assert RenderStyle.EDITABLE == EDITABLE_FIELDS
+    assert set(style.editable_values()) == set(EDITABLE_FIELDS)
+    # Every advertised field must exist on the dataclass with the right kind.
+    kinds = {name: kind for name, _label, kind in STYLE_FIELDS}
+    for name in EDITABLE_FIELDS:
+        assert hasattr(style, name), name
+        assert kinds[name] in {"color", "float", "int"}
+
+
+def test_response_colorscale_is_an_evenly_spaced_mapping() -> None:
+    style = RenderStyle()
+    scale = style.response_colorscale()
+    assert len(scale) == len(style.response_scale_colors)
+    assert scale[0] == (0.0, style.response_scale_colors[0])
+    assert scale[-1] == (1.0, style.response_scale_colors[-1])
+    positions = [position for position, _color in scale]
+    assert positions == sorted(positions)
+
+
+@pytest.mark.parametrize(
+    ("element", "expected"),
+    [
+        (ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1), 0),
+        (TrussElement(id=2, nodes=(1, 2), area=0.01, material_id=1), 1),
+        (ZeroLengthElement(id=3, nodes=(1, 1), material_ids=(1,), dofs=(1,)), 2),
+    ],
+)
+def test_element_family_index(element: object, expected: int) -> None:
+    assert element_family_index(element) == expected
+
+
+def test_family_palette_order_and_selected_slot() -> None:
+    style = RenderStyle(node_color="#010101")
+    palette = family_palette(style)
+    assert palette == [
+        style.element_beam_color,
+        style.element_truss_color,
+        style.element_link_color,
+        style.selected_color,
+    ]
+    assert len(palette) - 1 == SELECTED_STATE
+
+
+def test_editable_values_round_trip_through_replace() -> None:
+    style = RenderStyle()
+    values = style.editable_values()
+    assert RenderStyle(**values) == style
+    # And the persisted shape is JSON-friendly (str/int/float only).
+    import json
+
+    assert json.loads(json.dumps(values)) == values
+
+
+def test_style_is_a_frozen_slots_dataclass() -> None:
+    assert dataclasses.is_dataclass(RenderStyle)
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        RenderStyle().node_color = "#000000"  # type: ignore[misc]
+
+
+def test_project_builder_helper_is_unused_but_valid() -> None:
+    """A style constructed from a project's elements classifies every family."""
+    project = Project(
+        ndm=2,
+        ndf=3,
+        nodes=[Node(id=1, coords=(0, 0, 0)), Node(id=2, coords=(1, 0, 0))],
+        elements=[
+            ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1),
+            TrussElement(id=2, nodes=(1, 2), area=0.01, material_id=1),
+        ],
+    )
+    indices = [element_family_index(el) for el in project.elements]
+    assert indices == [0, 1]

tests/unit/test_renderer.py

@@ -24,6 +24,7 @@ from otko.views.canvas3d.model_renderer import (
     RendererMode,
     _classify_support,
 )
+from otko.views.canvas3d.style import SELECTED_STATE
 
 
 # ──────────────────────────── support classification ────────────────────────────
@@ -125,9 +126,13 @@ def test_update_selection_writes_state_array(offscreen_plotter, small_3d_project
     frame_states = np.asarray(r._frame_pd.cell_data["_oss_state"]).tolist()
     # Nodes 1 and 3 selected → row 0 and row 2
     assert node_states == [1, 0, 1]
-    # Element 2 selected → it's the second frame (index 1 in frame_ids_ordered)
+    # Frame cells carry the palette slot: the element family normally, and
+    # SELECTED_STATE when selected.
     selected_frame_idx = r._frame_id_to_row[2]
-    assert frame_states[selected_frame_idx] == 1
+    assert frame_states[selected_frame_idx] == SELECTED_STATE
+    for index, family in enumerate(r._frame_family):
+        expected = SELECTED_STATE if index == selected_frame_idx else family
+        assert frame_states[index] == expected
 
 
 def test_clear_selection(offscreen_plotter, small_3d_project) -> None:  # type: ignore[no-untyped-def]