feat: named case-result load combinations with full GUI support

Snapshots the current development tree, headlined by proper load
combinations (user request): a reusable LoadCombination entity of
weighted completed static-case results (e.g. 1.2xDead + 1.6xLive).

- core: LoadCombination/LoadCombinationItem entities, Project
  integration (lookup, unique ids, reference validation)
- services: combinations.py (linear superposition + envelope),
  exported via services __init__
- commands: undoable Add/Delete/Update for combinations
- GUI: Load Combinations manager dialog, Run-dialog evaluation,
  envelope display in Results panel, Combinations tab in Table dock
- tests: unit coverage (validation, math, error paths) + integration
  superposition check vs a single factored run

tests/gui/test_section_extrusion.py

@@ -0,0 +1,86 @@
+"""Smoke tests for the Show Extruded Sections overlay."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import (  # noqa: E402
+    ElasticBeamColumn,
+    ElasticSection,
+    Node,
+    Project,
+)
+
+
+def _frame_project() -> Project:
+    # Rectangular section 0.3×0.5 m.
+    b, h = 0.30, 0.50
+    A = b * h
+    Iz = b * h**3 / 12.0
+    Iy = h * b**3 / 12.0
+    return Project(
+        nodes=[
+            Node(id=1, coords=(0.0, 0.0, 0.0), restraint=(True, True, True, True, True, True)),
+            Node(id=2, coords=(6.0, 0.0, 0.0)),
+        ],
+        sections=[
+            ElasticSection(
+                id=1,
+                name="Rect",
+                E=200e9,
+                A=A,
+                Iz=Iz,
+                Iy=Iy,
+                G=80e9,
+                J=1e-6,
+            )
+        ],
+        elements=[
+            ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1),
+        ],
+    )
+
+
+@pytest.mark.gui
+def test_toggle_creates_and_removes_extrusion_actor(qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas3d.model_canvas import ModelCanvas
+
+    canvas = ModelCanvas()
+    qtbot.addWidget(canvas)
+    canvas.show_project(_frame_project())
+
+    before = len(canvas._renderer._aux_actors)
+    canvas.set_show_section_extrusions(True)
+    after_on = len(canvas._renderer._aux_actors)
+    assert after_on > before, "Extrusion actor should appear after toggling on"
+
+    canvas.set_show_section_extrusions(False)
+    after_off = len(canvas._renderer._aux_actors)
+    assert after_off == before, "Extrusion actor must disappear when toggled off"
+
+
+@pytest.mark.gui
+def test_extrusion_skips_elements_without_section(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """A truss element has no section — it must be skipped silently."""
+    from otko.core import ElasticUniaxial, TrussElement
+    from otko.views.canvas3d.model_canvas import ModelCanvas
+
+    canvas = ModelCanvas()
+    qtbot.addWidget(canvas)
+    p = Project(
+        nodes=[
+            Node(id=1, coords=(0, 0, 0), restraint=(True, True, True, True, True, True)),
+            Node(id=2, coords=(3, 0, 0)),
+        ],
+        materials=[ElasticUniaxial(id=1, E=200e9)],
+        elements=[
+            TrussElement(id=1, nodes=(1, 2), area=1e-3, material_id=1),
+        ],
+    )
+    canvas.show_project(p)
+    canvas.set_show_section_extrusions(True)
+    # No crash, no extra actor (truss has no section_id).
+    # (Other aux actors like the grid might exist; only verify nothing blew up.)
+    assert canvas._renderer._aux_actors is not None