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/integration/test_dof_coverage.py

@@ -0,0 +1,76 @@
+"""Regression test: pure-truss models with ndf=3 must refuse to solve.
+
+OpenSees silently returns the load vector as 'displacement' when the
+stiffness matrix is singular on rotational DOFs. The runner now
+pre-validates DOF coverage and raises a descriptive error instead.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("openseespy")
+
+from otko.core import (  # noqa: E402
+    ElasticUniaxial,
+    LinearTimeSeries,
+    NodalLoad,
+    Node,
+    PlainLoadPattern,
+    Project,
+    StaticCase,
+    TrussElement,
+)
+from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def _make_truss_project(ndf: int) -> Project:
+    return Project(
+        ndm=2,
+        ndf=ndf,
+        nodes=[
+            Node(id=1, coords=(0, 0, 0), restraint=(True, True, False, False, False, False)),
+            Node(id=2, coords=(3, 0, 0), restraint=(True, True, False, False, False, False)),
+            Node(id=3, coords=(1.5, 2, 0)),
+        ],
+        materials=[ElasticUniaxial(id=1, E=200e9)],
+        elements=[
+            TrussElement(id=1, nodes=(1, 3), area=1e-3, material_id=1),
+            TrussElement(id=2, nodes=(2, 3), area=1e-3, material_id=1),
+        ],
+        time_series=[LinearTimeSeries(id=1, name="R")],
+        load_patterns=[
+            PlainLoadPattern(
+                id=1,
+                time_series_id=1,
+                nodal_loads=[NodalLoad(node_id=3, forces=(1e3, -5e3, 0, 0, 0, 0))],
+            )
+        ],
+        analyses=[StaticCase(id=1, name="Static", pattern_ids=[1], n_steps=1)],
+    )
+
+
+def test_truss_with_ndf_3_is_rejected_clearly() -> None:
+    """A pure-truss project declared with ndf=3 must fail before the solve."""
+    proj = _make_truss_project(ndf=3)
+    with pytest.raises(RuntimeError) as excinfo:
+        OpenSeesRunner(proj).run(proj.analyses[0])
+    message = str(excinfo.value)
+    assert "Singular stiffness matrix" in message
+    # Every unrestrained Rz should be listed.
+    assert "Rz" in message
+    # Helpful hint steering the user to the right menu item.
+    assert "New 2D Truss" in message
+
+
+def test_truss_with_ndf_2_runs_to_completion() -> None:
+    """The same truss with ndf=2 solves fine."""
+    proj = _make_truss_project(ndf=2)
+    result = OpenSeesRunner(proj).run(proj.analyses[0])
+    assert 3 in result.node_disp
+    # Sanity: displacement must be non-zero and finite.
+    ux, uy = result.node_disp[3][-1]
+    assert abs(ux) > 0 or abs(uy) > 0
+    assert ux == ux and uy == uy  # NaN check