feat: opstool-derived element palette and single-trace frame colours

Ports opstool's per-family element colours and its diverging response
scale; RenderStyle.response_scale_colors now also drives the PyVista
force-diagram colouring instead of a hard-coded "coolwarm".

The frame renderer drops the two-trace normal/selected workaround: my
earlier assumption that plotly cannot colour segments individually was
wrong. Scatter3d.line.color accepts an array mapped through a colorscale,
so one trace now carries per-element colours (family + selection) and is
ready to be coloured by response value later.

Attribution recorded in NOTICE per GPLv3 section 5(a)/(b).

tests/unit/test_plotly_trace_builder.py

@@ -9,6 +9,7 @@ import numpy as np
 import pytest
 
 from otko.services import load_project
+from otko.views.canvas3d.style import RenderStyle
 from otko.views.canvas_plotly.trace_builder import (
     PlotlyTraceBuilder,
     SceneOptions,
@@ -49,16 +50,47 @@ def test_nodes_carry_ids_as_customdata() -> None:
     assert len(nodes["marker"]["color"]) == len(project.nodes)
 
 
-def test_selected_element_gets_its_own_trace() -> None:
+def _color_for_index(trace: dict, index: float) -> str:
+    """Map a ``line.color`` scalar index to its colour via the colorscale."""
+    scale = sorted(trace["line"]["colorscale"], key=lambda item: item[0])
+    return scale[int(index)][1]
+
+
+def test_frames_are_one_trace_with_per_element_colours() -> None:
+    """A single line trace carries the palette through its ``line.color`` array."""
     project = _load("cantilever")
-    scene = PlotlyTraceBuilder().build(project, SceneOptions(selection_elements=frozenset({2})))
-    normal = _traces(scene, "elements")
-    selected = _traces(scene, "elements-selected")
-    assert len(normal) == 1 and len(selected) == 1
-    # Plotly cannot colour one segment of a single line trace, so the
-    # selected element moves to the highlight trace.
-    assert 2 in selected[0]["customdata"]
-    assert 2 not in normal[0]["customdata"]
+    style = RenderStyle()
+    scene = PlotlyTraceBuilder(style).build(
+        project, SceneOptions(selection_elements=frozenset({2}))
+    )
+    frames = _traces(scene, "elements")
+    assert len(frames) == 1, "selection must not split the frame trace"
+    line = frames[0]["line"]
+    assert isinstance(line["color"], list)
+    assert len(line["color"]) == len(frames[0]["x"])
+    assert line["colorscale"]
+
+    colors = line["color"]
+    customdata = frames[0]["customdata"]
+
+    def index_of(element_id: int) -> set[float]:
+        return {colors[k] for k, value in enumerate(customdata) if value == element_id}
+
+    selected = index_of(2)
+    unselected = index_of(1)
+    assert len(selected) == 1 and len(unselected) == 1
+    assert selected != unselected
+    assert _color_for_index(frames[0], next(iter(selected))) == style.selected_color
+    assert _color_for_index(frames[0], next(iter(unselected))) == style.element_beam_color
+
+
+def test_element_palette_distinguishes_beam_from_truss() -> None:
+    style = RenderStyle()
+    builder = PlotlyTraceBuilder(style)
+    beam = _traces(builder.build(_load("cantilever"), SceneOptions()), "elements")[0]
+    truss = _traces(builder.build(_load("basic_truss"), SceneOptions()), "elements")[0]
+    assert _color_for_index(beam, beam["line"]["color"][0]) == style.element_beam_color
+    assert _color_for_index(truss, truss["line"]["color"][0]) == style.element_truss_color
 
 
 def test_selected_node_is_highlighted_by_colour() -> None: