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_static.py

@@ -0,0 +1,70 @@
+"""Static analysis verification.
+
+Cantilever beam: tip lateral load → tip displacement δ = P L³ / (3 E I).
+Compares the runner's static result against the closed-form solution.
+"""
+
+from __future__ import annotations
+
+import math
+
+import pytest
+
+ops = pytest.importorskip("openseespy.opensees")  # skip if OpenSeesPy not installed
+
+from otko.core import (  # noqa: E402
+    ElasticBeamColumn,
+    ElasticSection,
+    LinearTimeSeries,
+    NodalLoad,
+    Node,
+    PlainLoadPattern,
+    Project,
+    StaticCase,
+)
+from otko.services import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def test_cantilever_tip_deflection_matches_closed_form() -> None:
+    """Horizontal cantilever in 2D: P at tip, expect δ = P L³ / (3 E I)."""
+    L = 5.0
+    P = 1000.0
+    E = 200e9
+    A = 0.01
+    I = 8.333e-6
+
+    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=(L, 0.0, 0.0)),
+        ],
+        sections=[ElasticSection(id=1, E=E, A=A, Iz=I)],
+        elements=[ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1)],
+        time_series=[LinearTimeSeries(id=1)],
+        load_patterns=[
+            PlainLoadPattern(
+                id=1,
+                time_series_id=1,
+                # Fy at the tip (downward); Rz of node 1 is restrained, others free.
+                nodal_loads=[NodalLoad(node_id=2, forces=(0.0, -P, 0.0, 0.0, 0.0, 0.0))],
+            )
+        ],
+    )
+
+    case = StaticCase(id=1, name="Cantilever", pattern_ids=[1])
+    results = OpenSeesRunner(project).run(case)
+
+    delta_expected = -P * L**3 / (3.0 * E * I)  # negative (downward)
+    delta_actual = results.disp(node_id=2, dof=2)  # Uy at node 2
+
+    assert math.isclose(
+        delta_actual, delta_expected, rel_tol=1e-3
+    ), f"Tip deflection mismatch: expected {delta_expected:.6e}, got {delta_actual:.6e}"
+
+    # Reaction at the support equals the applied load.
+    Fy_reaction = results.node_reaction[1][0, 1]
+    assert math.isclose(Fy_reaction, P, rel_tol=1e-6)