feat: live canvas backend switching (Options → Canvas Backend)

Both backends share one SelectionState owned by MainWindow and live
side by side in a QStackedWidget — switching is setCurrentWidget, so
no widget is destroyed mid-session (tearing a VTK window down leaves
dangling make-current callbacks). The choice persists in QSettings.
CanvasCapabilities declares per-backend gaps (force diagrams and
video export stay PyVista-only, both are documented and greyed out)
and the UI gates on capabilities rather than the backend name. The
architecture gate now allows the canvas_plotly package.

tests/gui/test_canvas_backend_switch.py

@@ -0,0 +1,122 @@
+"""GUI tests for the live canvas-backend switch (PyVista ⇄ Plotly).
+
+QSettings are injected per-test so a swap never leaks into the real user
+configuration or the next test.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("pyvistaqt")
+pytest.importorskip("plotly")
+
+from PySide6.QtCore import QSettings
+
+
+def _make_window(tmp_path, qtbot):  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    settings = QSettings(str(tmp_path / "otko.ini"), QSettings.Format.IniFormat)
+    window = MainWindow(settings=settings)
+    qtbot.addWidget(window)
+    return window
+
+
+@pytest.mark.gui
+def test_default_backend_is_pyvista(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas3d import ModelCanvas
+
+    window = _make_window(tmp_path, qtbot)
+    assert window.canvas_backend() == "pyvista"
+    assert isinstance(window._canvas, ModelCanvas)
+    assert window._diagram_renderer is not None
+
+
+@pytest.mark.gui
+def test_switch_to_plotly_preserves_selection(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas_plotly import PlotlyCanvas
+
+    window = _make_window(tmp_path, qtbot)
+    window._vm.new_project()
+    window._canvas.selection.select_node(2)
+
+    window.swap_canvas_backend("plotly")
+
+    assert window.canvas_backend() == "plotly"
+    assert isinstance(window._canvas, PlotlyCanvas)
+    # One shared SelectionState: the tree/table/properties stay bound.
+    assert window._canvas.selection is window._selection
+    assert window._canvas.selection.nodes == frozenset({2})
+    # Both canvases live in the stack; the PyVista one was not destroyed.
+    assert window._canvas_stack.count() == 2
+
+
+@pytest.mark.gui
+def test_plotly_gates_pyvista_only_features(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    assert window._canvas.capabilities.diagrams is True
+
+    window.swap_canvas_backend("plotly")
+    caps = window._canvas.capabilities
+    assert caps.diagrams is False
+    assert caps.animation_export is False
+    assert caps.extrusions is True and caps.labels is True
+
+    window._refresh_action_enablement()
+    assert window._act_show_force_diagram.isEnabled() is False
+    assert window._act_export_th_animation.isEnabled() is False
+
+
+@pytest.mark.gui
+def test_swapping_back_reuses_the_same_canvases(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    pyvista_canvas = window._canvas
+
+    window.swap_canvas_backend("plotly")
+    plotly_canvas = window._canvas
+    window.swap_canvas_backend("pyvista")
+
+    assert window._canvas is pyvista_canvas
+    assert window._diagram_renderer is not None
+    # No duplicate widgets accumulate across repeated swaps.
+    window.swap_canvas_backend("plotly")
+    assert window._canvas is plotly_canvas
+    assert window._diagram_renderer is None  # diagrams are PyVista-only
+    assert window._canvas_stack.count() == 2
+
+
+@pytest.mark.gui
+def test_backend_choice_is_persisted(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    window.swap_canvas_backend("plotly")
+    assert window._settings.value("canvas/backend") == "plotly"
+
+    # A fresh window with the same settings boots straight into Plotly.
+    from otko.views.canvas_plotly import PlotlyCanvas
+    from otko.views.main_window import MainWindow
+
+    settings = QSettings(str(tmp_path / "otko.ini"), QSettings.Format.IniFormat)
+    second = MainWindow(settings=settings)
+    qtbot.addWidget(second)
+    assert second.canvas_backend() == "plotly"
+    assert isinstance(second._canvas, PlotlyCanvas)
+
+
+@pytest.mark.gui
+def test_working_plane_survives_the_swap(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    window._vm.open(_example("basic_truss"))
+    window._canvas.set_working_plane("XY", 0.0)
+
+    window.swap_canvas_backend("plotly")
+
+    assert window._canvas.working_plane_type() == "XY"
+    assert window._canvas.working_plane_offset() == 0.0
+
+
+def _example(name: str) -> str:
+    from pathlib import Path
+
+    return str(Path(__file__).resolve().parents[2] / "examples" / f"{name}.osmodel")