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_constraint_and_display_dialogs.py

@@ -0,0 +1,40 @@
+"""GUI tests for equalDOF and display-option dialogs."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.views.dialogs.assign_equal_dof import AssignEqualDOFDialog
+from otko.views.dialogs.display_options import DisplayOptionsDialog
+
+
+@pytest.mark.gui
+def test_assign_equal_dof_dialog_requires_explicit_dof_choice(qtbot) -> None:  # type: ignore[no-untyped-def]
+    dlg = AssignEqualDOFDialog([3, 4], ndf=3)
+    qtbot.addWidget(dlg)
+
+    with pytest.raises(ValueError, match="at least one DOF"):
+        dlg.constraint()
+
+    dlg2 = AssignEqualDOFDialog([3, 4], ndf=3, suggested_dofs=(2, 3))
+    qtbot.addWidget(dlg2)
+
+    c = dlg2.constraint()
+    assert c.retained_node == 3
+    assert c.constrained_node == 4
+    assert c.dofs == (2, 3)
+
+
+@pytest.mark.gui
+def test_display_options_dialog_round_trip(qtbot) -> None:  # type: ignore[no-untyped-def]
+    dlg = DisplayOptionsDialog(
+        show_node_labels=False,
+        show_element_labels=True,
+    )
+    qtbot.addWidget(dlg)
+
+    dlg._show_node_labels.setChecked(True)
+    dlg._show_element_labels.setChecked(False)
+    assert dlg.values() == (True, False)