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_update_element_fields.py

@@ -0,0 +1,99 @@
+"""Tests for UpdateElementFieldsCommand — inline scalar edits."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.commands import (  # noqa: E402
+    AddElementsCommand,
+    AddMaterialsCommand,
+    AddNodesCommand,
+    UpdateElementFieldsCommand,
+)
+from otko.core import (  # noqa: E402
+    ElasticUniaxial,
+    Node,
+    TrussElement,
+)
+from otko.viewmodels import ProjectViewModel  # noqa: E402
+
+
+def _vm_with_truss() -> ProjectViewModel:
+    vm = ProjectViewModel()
+    vm.new_project()
+    vm.apply_command(
+        AddNodesCommand(
+            vm,
+            [
+                Node(id=1, coords=(0, 0, 0)),
+                Node(id=2, coords=(3, 0, 0)),
+            ],
+        )
+    )
+    vm.apply_command(
+        AddMaterialsCommand(
+            vm,
+            [
+                ElasticUniaxial(id=1, name="Steel", E=200e9),
+            ],
+        )
+    )
+    vm.apply_command(
+        AddElementsCommand(
+            vm,
+            [
+                TrussElement(id=1, nodes=(1, 2), area=0.001, material_id=1),
+            ],
+        )
+    )
+    return vm
+
+
+@pytest.mark.gui
+def test_update_area_on_truss_element(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm_with_truss()
+    vm.apply_command(UpdateElementFieldsCommand(vm, 1, {"area": 0.00645}))
+    assert vm.project.element(1).area == pytest.approx(0.00645)
+
+
+@pytest.mark.gui
+def test_update_area_is_undoable(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm_with_truss()
+    vm.apply_command(UpdateElementFieldsCommand(vm, 1, {"area": 0.005}))
+    vm.undo_stack.undo()
+    assert vm.project.element(1).area == pytest.approx(0.001)
+    vm.undo_stack.redo()
+    assert vm.project.element(1).area == pytest.approx(0.005)
+
+
+@pytest.mark.gui
+def test_unknown_field_is_silently_ignored(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Fields not in the element's model are filtered out — no exception."""
+    vm = _vm_with_truss()
+    vm.apply_command(
+        UpdateElementFieldsCommand(
+            vm,
+            1,
+            {"section_id": 42},  # TrussElement has no section_id
+        )
+    )
+    # The element is unchanged.
+    assert vm.project.element(1).area == pytest.approx(0.001)
+    assert vm.project.element(1).material_id == 1
+
+
+@pytest.mark.gui
+def test_update_multiple_fields_at_once(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm_with_truss()
+    vm.apply_command(
+        UpdateElementFieldsCommand(
+            vm,
+            1,
+            {"area": 0.003, "rho": 7850.0},
+        )
+    )
+    el = vm.project.element(1)
+    assert el.area == pytest.approx(0.003)
+    assert el.rho == pytest.approx(7850.0)