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_commands.py

@@ -0,0 +1,181 @@
+"""Unit tests for ProjectCommand subclasses.
+
+Verify that every command's ``redo`` is exactly reversed by ``undo``,
+that the undo stack auto-tracks dirty state, and that cascade-delete
+removes affected elements.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.commands import (  # noqa: E402
+    AddElementsCommand,
+    AddNodalLoadsCommand,
+    AddNodesCommand,
+    DeleteElementsCommand,
+    DeleteNodesCommand,
+    SetRestraintCommand,
+)
+from otko.core import (  # noqa: E402
+    LinearTimeSeries,
+    NodalLoad,
+    Node,
+    PlainLoadPattern,
+    Steel01,
+    TrussElement,
+)
+from otko.viewmodels import ProjectViewModel  # noqa: E402
+
+
+# ─────────────────────────── helpers ────────────────────────────────
+def _vm() -> ProjectViewModel:
+    vm = ProjectViewModel()
+    vm.new_project()
+    return vm
+
+
+# ─────────────────────────── AddNodesCommand ────────────────────────
+@pytest.mark.gui
+def test_add_nodes_then_undo(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    new_nodes = [Node(id=1, coords=(0, 0, 0)), Node(id=2, coords=(1, 0, 0))]
+    vm.apply_command(AddNodesCommand(vm, new_nodes))
+    assert len(vm.project.nodes) == 2
+    vm.undo_stack.undo()
+    assert len(vm.project.nodes) == 0
+    vm.undo_stack.redo()
+    assert len(vm.project.nodes) == 2
+
+
+@pytest.mark.gui
+def test_add_nodes_emits_modelMutated(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    with qtbot.waitSignal(vm.modelMutated, timeout=500):
+        vm.apply_command(AddNodesCommand(vm, [Node(id=1, coords=(0, 0, 0))]))
+
+
+@pytest.mark.gui
+def test_add_nodes_marks_dirty(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    assert not vm.is_dirty
+    vm.apply_command(AddNodesCommand(vm, [Node(id=1, coords=(0, 0, 0))]))
+    assert vm.is_dirty
+
+
+@pytest.mark.gui
+def test_add_nodes_duplicate_id_raises(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(AddNodesCommand(vm, [Node(id=5, coords=(0, 0, 0))]))
+    with pytest.raises(ValueError, match="already exists"):
+        vm.apply_command(AddNodesCommand(vm, [Node(id=5, coords=(1, 0, 0))]))
+
+
+# ─────────────────────────── DeleteNodesCommand (cascade) ───────────
+@pytest.mark.gui
+def test_delete_node_cascades_to_elements(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(
+        AddNodesCommand(
+            vm,
+            [
+                Node(id=1, coords=(0, 0, 0)),
+                Node(id=2, coords=(1, 0, 0)),
+                Node(id=3, coords=(2, 0, 0)),
+            ],
+        )
+    )
+    # Add a material so the element is valid.
+    vm.project.materials.append(Steel01(id=1, Fy=420e6, E0=200e9, b=0.01))
+    vm.apply_command(
+        AddElementsCommand(
+            vm,
+            [
+                TrussElement(id=1, nodes=(1, 2), area=1e-3, material_id=1),
+                TrussElement(id=2, nodes=(2, 3), area=1e-3, material_id=1),
+            ],
+        )
+    )
+    # Delete node 2 → both elements should disappear.
+    vm.apply_command(DeleteNodesCommand(vm, {2}))
+    assert {n.id for n in vm.project.nodes} == {1, 3}
+    assert vm.project.elements == []
+    # Undo restores everything.
+    vm.undo_stack.undo()
+    assert {n.id for n in vm.project.nodes} == {1, 2, 3}
+    assert {e.id for e in vm.project.elements} == {1, 2}
+
+
+# ─────────────────────────── SetRestraintCommand ───────────────────
+@pytest.mark.gui
+def test_set_restraint_round_trip(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(
+        AddNodesCommand(
+            vm,
+            [
+                Node(id=1, coords=(0, 0, 0)),
+                Node(id=2, coords=(1, 0, 0)),
+            ],
+        )
+    )
+    fix = (True, True, True, True, True, True)
+    vm.apply_command(SetRestraintCommand(vm, {1, 2}, fix))
+    assert vm.project.node(1).restraint == fix
+    assert vm.project.node(2).restraint == fix
+    vm.undo_stack.undo()
+    assert vm.project.node(1).restraint == (False,) * 6
+    assert vm.project.node(2).restraint == (False,) * 6
+
+
+# ─────────────────────────── AddNodalLoadsCommand ──────────────────
+@pytest.mark.gui
+def test_add_load_creates_default_pattern(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(AddNodesCommand(vm, [Node(id=1, coords=(0, 0, 0))]))
+    forces = (100.0, 0, 0, 0, 0, 0)
+    vm.apply_command(AddNodalLoadsCommand(vm, {1}, forces))
+    assert len(vm.project.time_series) == 1
+    assert isinstance(vm.project.time_series[0], LinearTimeSeries)
+    assert len(vm.project.load_patterns) == 1
+    pat = vm.project.load_patterns[0]
+    assert isinstance(pat, PlainLoadPattern)
+    assert len(pat.nodal_loads) == 1
+    assert pat.nodal_loads[0] == NodalLoad(node_id=1, forces=forces)
+
+
+@pytest.mark.gui
+def test_add_load_undo_removes_default_pattern(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(AddNodesCommand(vm, [Node(id=1, coords=(0, 0, 0))]))
+    vm.apply_command(AddNodalLoadsCommand(vm, {1}, (100.0, 0, 0, 0, 0, 0)))
+    vm.undo_stack.undo()
+    # Both the load AND the auto-created pattern + ts should be gone.
+    assert vm.project.time_series == []
+    assert vm.project.load_patterns == []
+
+
+@pytest.mark.gui
+def test_add_load_uses_existing_pattern(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(AddNodesCommand(vm, [Node(id=1, coords=(0, 0, 0))]))
+    # Pre-populate a pattern.
+    vm.project.time_series.append(LinearTimeSeries(id=1))
+    vm.project.load_patterns.append(PlainLoadPattern(id=1, time_series_id=1))
+    vm.apply_command(AddNodalLoadsCommand(vm, {1}, (50.0, 0, 0, 0, 0, 0)))
+    # No new pattern should be created.
+    assert len(vm.project.time_series) == 1
+    assert len(vm.project.load_patterns) == 1
+    assert len(vm.project.load_patterns[0].nodal_loads) == 1
+
+
+# ─────────────────────────── stack semantics ───────────────────────
+@pytest.mark.gui
+def test_save_marks_clean(qtbot, tmp_path) -> None:  # type: ignore[no-untyped-def]
+    vm = _vm()
+    vm.apply_command(AddNodesCommand(vm, [Node(id=1, coords=(0, 0, 0))]))
+    assert vm.is_dirty
+    vm.save(tmp_path / "x.osmodel")
+    assert not vm.is_dirty