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_runner_modal.py

@@ -0,0 +1,90 @@
+"""Modal analysis verification.
+
+A 1-DOF lumped-mass cantilever pole. The fundamental natural frequency
+of the lateral mode is ω = √(k/m), where k = 3EI/L³ for a tip-mass
+cantilever flexural spring.
+
+The runner auto-falls back to ``-fullGenLapack`` for small models —
+ARPACK can't allocate enough Arnoldi workspace when the active DOF
+count is tiny, which is exactly the SDOF case.
+"""
+
+from __future__ import annotations
+
+import math
+
+import pytest
+
+ops = pytest.importorskip("openseespy.opensees")  # noqa: F401
+
+from otko.core import (  # noqa: E402
+    ElasticBeamColumn,
+    ElasticSection,
+    ModalCase,
+    Node,
+    Project,
+)
+from otko.services import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def test_sdof_pole_first_frequency_matches_kspring_over_m() -> None:
+    """Vertical pole, mass at top, fixed base. ω₁ = √(3EI/(mL³))."""
+    L = 3.0
+    E = 200e9
+    A = 0.01
+    I = 8.333e-6
+    m_tip = 1000.0
+
+    project = Project(
+        ndm=2,
+        ndf=3,
+        nodes=[
+            Node(id=1, coords=(0.0, 0.0, 0.0), restraint=(True, True, False, False, False, True)),
+            Node(id=2, coords=(0.0, L, 0.0), mass=(m_tip, m_tip, 0.0, 0.0, 0.0, 0.0)),
+        ],
+        sections=[ElasticSection(id=1, E=E, A=A, Iz=I)],
+        elements=[ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1)],
+    )
+
+    case = ModalCase(id=1, name="SDOF-Pole", n_modes=1)
+    results = OpenSeesRunner(project).run(case)
+
+    # Lateral cantilever spring stiffness:
+    k = 3.0 * E * I / L**3
+    omega_expected = math.sqrt(k / m_tip)
+
+    omega_actual = float(results.angular_frequencies[0])
+    assert math.isclose(
+        omega_actual, omega_expected, rel_tol=5e-3
+    ), f"ω₁ mismatch: expected {omega_expected:.4f} rad/s, got {omega_actual:.4f} rad/s"
+
+
+def test_runner_falls_back_to_lapack_for_small_models() -> None:
+    """Verify the fallback rule: small n_free triggers Lapack instead of ARPACK."""
+    from unittest.mock import MagicMock
+
+    project = Project(
+        ndm=2,
+        ndf=3,
+        nodes=[
+            Node(id=1, coords=(0.0, 0.0, 0.0), restraint=(True, True, False, False, False, True)),
+            Node(id=2, coords=(0.0, 3.0, 0.0), mass=(1000.0, 1000.0, 0.0, 0.0, 0.0, 0.0)),
+        ],
+        sections=[ElasticSection(id=1, E=200e9, A=0.01, Iz=8.333e-6)],
+        elements=[ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1)],
+    )
+    mock_ops = MagicMock()
+    mock_ops.eigen.return_value = [1.0]
+    mock_ops.nodeEigenvector.return_value = 0.0
+
+    runner = OpenSeesRunner(project, ops_module=mock_ops)
+    runner.run(ModalCase(id=1, name="t", n_modes=1))  # default solver = 'genBandArpack'
+
+    # n_free = (3 - 2) + (3 - 1) = 3, and 2 * n_modes = 2 >= n_free? No, 2 < 3.
+    # Adjust: ask for 2 modes — 2*2 = 4 >= 3 → must trigger fallback.
+    mock_ops.reset_mock()
+    mock_ops.eigen.return_value = [1.0, 4.0]
+    runner.run(ModalCase(id=2, name="t2", n_modes=2))
+    mock_ops.eigen.assert_called_with("-fullGenLapack", 2)