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_draw_truss_tool.py

@@ -0,0 +1,121 @@
+"""Tests for DrawTrussTool — two-click truss-bar drawing."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import (  # noqa: E402
+    CoordinateGridSystem,
+    CoordinateSystem,
+    ElasticUniaxial,
+    GridSystem,
+    Node,
+    TrussElement,
+    make_grid_lines,
+)
+from otko.viewmodels import ProjectViewModel  # noqa: E402
+from otko.views.canvas3d.selection import SelectionState  # noqa: E402
+from otko.views.tools.draw_truss import DrawTrussTool  # noqa: E402
+
+
+class _CanvasStub:
+    def __init__(self) -> None:
+        self.selection = SelectionState()
+
+    def view_xy(self) -> None:
+        pass
+
+    def set_snap_preview_enabled(self, _enabled: bool) -> None:
+        pass
+
+
+def _vm_with_grid_and_nodes() -> ProjectViewModel:
+    vm = ProjectViewModel()
+    vm.new_project()
+    vm.project.nodes.extend(
+        [
+            Node(id=1, coords=(0, 0, 0)),
+            Node(id=2, coords=(3, 0, 0)),
+        ]
+    )
+    vm.project.coord_systems = [
+        CoordinateGridSystem(
+            name="Global",
+            grid=GridSystem(
+                x_grid_lines=make_grid_lines("X", [0.0, 3.0]),
+                y_grid_lines=make_grid_lines("Y", [0.0]),
+                z_grid_lines=make_grid_lines("Z", [0.0]),
+            ),
+        ),
+    ]
+    return vm
+
+
+@pytest.mark.gui
+def test_two_clicks_create_truss(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm_with_grid_and_nodes()
+    tool = DrawTrussTool(_CanvasStub(), vm)  # type: ignore[arg-type]
+    tool.activate()
+
+    tool.on_node_picked(1)
+    tool.on_node_picked(2)
+
+    assert len(vm.project.elements) == 1  # type: ignore[union-attr]
+    el = vm.project.elements[0]  # type: ignore[union-attr]
+    assert isinstance(el, TrussElement)
+    assert el.nodes == (1, 2)
+    assert el.area > 0
+    # A default ElasticUniaxial must have been created for the material.
+    assert len(vm.project.materials) == 1  # type: ignore[union-attr]
+    assert isinstance(vm.project.materials[0], ElasticUniaxial)
+
+
+@pytest.mark.gui
+def test_draw_truss_reuses_existing_material(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm_with_grid_and_nodes()
+    vm.project.materials.append(ElasticUniaxial(id=1, name="Existing", E=150e9))  # type: ignore[union-attr]
+    tool = DrawTrussTool(_CanvasStub(), vm)  # type: ignore[arg-type]
+    tool.activate()
+    tool.on_node_picked(1)
+    tool.on_node_picked(2)
+    el = vm.project.elements[0]  # type: ignore[union-attr]
+    assert el.material_id == 1
+    assert len(vm.project.materials) == 1  # type: ignore[union-attr]  (no new material)
+
+
+@pytest.mark.gui
+def test_empty_click_creates_node_then_truss(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Canvas-assumed: coordinates passed are already on a snap target."""
+    vm = ProjectViewModel()
+    vm.new_project()
+    vm.project.coord_systems = [
+        CoordinateGridSystem(
+            name="Global",
+            grid=GridSystem(
+                x_grid_lines=make_grid_lines("X", [0.0, 3.0]),
+                y_grid_lines=make_grid_lines("Y", [0.0]),
+                z_grid_lines=make_grid_lines("Z", [0.0]),
+            ),
+        ),
+    ]
+    tool = DrawTrussTool(_CanvasStub(), vm)  # type: ignore[arg-type]
+    tool.activate()
+    tool.on_empty_clicked(0.0, 0.0, 0.0)
+    tool.on_empty_clicked(3.0, 0.0, 0.0)
+
+    # Two nodes created + one truss element.
+    assert len(vm.project.nodes) == 2  # type: ignore[union-attr]
+    assert len(vm.project.elements) == 1  # type: ignore[union-attr]
+    assert isinstance(vm.project.elements[0], TrussElement)
+
+
+@pytest.mark.gui
+def test_self_pick_is_ignored(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm_with_grid_and_nodes()
+    tool = DrawTrussTool(_CanvasStub(), vm)  # type: ignore[arg-type]
+    tool.activate()
+    tool.on_node_picked(1)
+    tool.on_node_picked(1)  # same node — must not create a zero-length truss
+    assert vm.project.elements == []  # type: ignore[union-attr]