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_viewport_axes.py

@@ -0,0 +1,107 @@
+"""Viewport orientation triad is present and survives every re-render path.
+
+Before this change the 3D viewport had no X/Y/Z corner marker — users
+who rotated the camera off the default isometric view lost track of
+which world axis was which. The triad is now built once during the
+canvas's scene-furniture setup and lives in screen-space via PyVista's
+``vtkOrientationMarkerWidget``, so it is not affected by the regular
+actor rebuilds in :meth:`ModelRenderer.render` /
+:meth:`ModelRenderer._teardown_all`.
+
+These tests pin the contract:
+
+- Triad exists right after ``MainWindow`` construction.
+- It still exists after a tree refresh (which calls
+  ``ModelRenderer.render`` through ``MainWindow._on_project_changed``).
+- It still exists after loading an empty project, then a small project
+  with a node — the most common real-world re-render path.
+- The triad carries the conventional X / Y / Z labels.
+- It is pinned to the lower-left viewport corner.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import Node
+
+
+@pytest.mark.gui
+def test_axes_widget_exists_after_window_construction(qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    aw = mw._canvas.renderer.axes_widget
+    assert aw is not None
+    # PyVista binds a vtkOrientationMarkerWidget; the orientation marker
+    # it carries is a vtkAxesActor with X/Y/Z label text.
+    actor = aw.GetOrientationMarker()
+    assert actor.GetXAxisLabelText() == "X"
+    assert actor.GetYAxisLabelText() == "Y"
+    assert actor.GetZAxisLabelText() == "Z"
+    # Pinned to the lower-left viewport patch (normalized 0..1 coords).
+    vp = aw.GetViewport()
+    assert vp[0] == pytest.approx(0.0)
+    assert vp[1] == pytest.approx(0.0)
+
+
+@pytest.mark.gui
+def test_axes_widget_survives_tree_refresh(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """``MainWindow._refresh_tree`` triggers a renderer rebuild via
+    ``_on_project_changed``; the orientation widget must stay put."""
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    aw_before = mw._canvas.renderer.axes_widget
+
+    mw._vm.new_project(ndm=3, ndf=6)
+    mw._vm.project.nodes.extend(
+        [
+            Node(id=1, coords=(0, 0, 0)),
+            Node(id=2, coords=(3, 0, 0)),
+        ]
+    )
+    mw._refresh_tree(mw._vm.project)
+
+    aw_after = mw._canvas.renderer.axes_widget
+    # Same widget instance — proves the marker survived the rebuild.
+    assert aw_after is aw_before
+    actor = aw_after.GetOrientationMarker()
+    assert actor.GetXAxisLabelText() == "X"
+    assert actor.GetYAxisLabelText() == "Y"
+    assert actor.GetZAxisLabelText() == "Z"
+
+
+@pytest.mark.gui
+def test_axes_widget_survives_empty_then_loaded_project(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """The two re-render paths the engineer hits during a session:
+    empty project → first node added → tree refreshed. The triad must
+    still be in the lower-left corner with all three labels."""
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    aw_before = mw._canvas.renderer.axes_widget
+    assert aw_before is not None
+
+    # Empty project — model renderer builds no actors, just the grid.
+    mw._vm.new_project(ndm=3, ndf=6)
+    mw._refresh_tree(mw._vm.project)
+    assert mw._canvas.renderer.axes_widget is aw_before
+
+    # Small project with one node — exercises the node-actor rebuild path.
+    mw._vm.project.nodes.append(Node(id=1, coords=(1, 2, 3)))
+    mw._refresh_tree(mw._vm.project)
+    assert mw._canvas.renderer.axes_widget is aw_before
+
+    # Same after a full canvas refresh too — show_project rebuilds actors.
+    mw._canvas.show_project(mw._vm.project)
+    assert mw._canvas.renderer.axes_widget is aw_before
+    actor = mw._canvas.renderer.axes_widget.GetOrientationMarker()
+    assert actor.GetXAxisLabelText() == "X"
+    assert actor.GetYAxisLabelText() == "Y"
+    assert actor.GetZAxisLabelText() == "Z"