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_pattern_factors.py

@@ -0,0 +1,65 @@
+"""Pattern-factor integration: scaled static load doubles the tip displacement."""
+
+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 _cantilever() -> Project:
+    length = 5.0
+    load = 1000.0
+    elastic_mod = 200e9
+    area = 0.01
+    inertia = 8.333e-6
+    return 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=(length, 0.0, 0.0)),
+        ],
+        sections=[ElasticSection(id=1, E=elastic_mod, A=area, Iz=inertia)],
+        elements=[ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1)],
+        time_series=[LinearTimeSeries(id=1)],
+        load_patterns=[
+            PlainLoadPattern(
+                id=1,
+                time_series_id=1,
+                nodal_loads=[NodalLoad(node_id=2, forces=(0.0, -load, 0.0, 0.0, 0.0, 0.0))],
+            )
+        ],
+    )
+
+
+def test_pattern_factor_doubles_tip_disp() -> None:
+    """StaticCase pattern_factors={1: 2.0} must double the tip displacement."""
+    base = OpenSeesRunner(_cantilever()).run(StaticCase(id=1, name="base", pattern_ids=[1]))
+    scaled = OpenSeesRunner(_cantilever()).run(
+        StaticCase(id=1, name="scaled", pattern_ids=[1], pattern_factors={1: 2.0})
+    )
+    d_base = base.disp(node_id=2, dof=2)
+    d_scaled = scaled.disp(node_id=2, dof=2)
+    assert d_base != 0.0
+    assert math.isclose(d_scaled, 2.0 * d_base, rel_tol=1e-9)