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_ex3_canti2d_variants.py

@@ -0,0 +1,91 @@
+"""Integration tests for OpenSees Example 3 cantilever build variants."""
+
+from __future__ import annotations
+
+import tempfile
+from pathlib import Path
+
+import pytest
+
+pytest.importorskip("openseespy")
+
+from otko.core import PushoverCase, TransientCase  # noqa: E402
+from otko.services import load_project, save_project  # noqa: E402
+from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def _reload(proj, tmp_path, stem: str):  # type: ignore[no-untyped-def]
+    path = tmp_path / f"{stem}.osmodel"
+    save_project(proj, path)
+    reloaded = load_project(path)
+    reloaded.validate_references()
+    return reloaded
+
+
+@pytest.mark.parametrize(
+    ("builder_name", "module_name", "nonlinear"),
+    [
+        ("build_ex3_canti2d_elastic_element", "examples.ex3_canti2d_elastic_element", False),
+        ("build_ex3_canti2d_inelastic_section", "examples.ex3_canti2d_inelastic_section", True),
+        (
+            "build_ex3_canti2d_inelastic_fiber_section",
+            "examples.ex3_canti2d_inelastic_fiber_section",
+            True,
+        ),
+    ],
+)
+def test_ex3_variant_pushover_runs(
+    tmp_path, builder_name: str, module_name: str, nonlinear: bool
+) -> None:  # type: ignore[no-untyped-def]
+    mod = __import__(module_name, fromlist=[builder_name, "PUSH_STEP", "PUSH_TARGET"])
+    proj = _reload(getattr(mod, builder_name)(), tmp_path, builder_name)
+    push_case = next(c for c in proj.analyses if isinstance(c, PushoverCase))
+    result = OpenSeesRunner(proj).run(push_case)
+
+    expected_pts = int(mod.PUSH_TARGET / mod.PUSH_STEP) + 1
+    assert len(result.control_disp) == expected_pts
+    assert result.control_disp[-1] == pytest.approx(mod.PUSH_TARGET, rel=1e-6)
+    assert max(result.base_shear) > 0.0
+
+    early = (result.base_shear[5] - result.base_shear[0]) / (
+        result.control_disp[5] - result.control_disp[0]
+    )
+    late = (result.base_shear[-1] - result.base_shear[-6]) / (
+        result.control_disp[-1] - result.control_disp[-6]
+    )
+    if nonlinear:
+        assert early > 2.0 * late
+    else:
+        assert early == pytest.approx(late, rel=0.02)
+
+
+@pytest.mark.parametrize(
+    ("builder_name", "module_name"),
+    [
+        ("build_ex3_canti2d_elastic_element", "examples.ex3_canti2d_elastic_element"),
+        ("build_ex3_canti2d_inelastic_section", "examples.ex3_canti2d_inelastic_section"),
+        (
+            "build_ex3_canti2d_inelastic_fiber_section",
+            "examples.ex3_canti2d_inelastic_fiber_section",
+        ),
+    ],
+)
+def test_ex3_variant_earthquake_runs(tmp_path, builder_name: str, module_name: str) -> None:  # type: ignore[no-untyped-def]
+    mod = __import__(module_name, fromlist=[builder_name, "ANALYSIS_DT", "ANALYSIS_STEPS"])
+    proj = _reload(getattr(mod, builder_name)(), tmp_path, f"{builder_name}_eq")
+    eq_case = next(c for c in proj.analyses if isinstance(c, TransientCase))
+
+    results_dir = Path(tempfile.mkdtemp(prefix=f"{builder_name}_"))
+    result = OpenSeesRunner(proj).run(eq_case, results_dir=results_dir)
+    time = result.time()
+    top = result.node_disp_history(2)
+    ux = top[:, 0]
+    uy = top[:, 1]
+
+    assert len(time) == mod.ANALYSIS_STEPS
+    assert result.dt == pytest.approx(mod.ANALYSIS_DT)
+    assert ux.max() > 1e-4
+    assert ux.min() < -1e-4
+    assert max(abs(uy)) < 1.0