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_ex1a_canti2d_eq.py

@@ -0,0 +1,53 @@
+"""Integration test for the 2D elastic cantilever earthquake example."""
+
+from __future__ import annotations
+
+import tempfile
+from pathlib import Path
+
+import pytest
+
+pytest.importorskip("openseespy")
+
+from otko.services import load_project, save_project  # noqa: E402
+from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def test_ex1a_canti2d_eq_runs_and_oscillates(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    from examples.ex1a_canti2d_eq import (
+        ANALYSIS_DT,
+        COLUMN_HEIGHT,
+        build_ex1a_canti2d_eq,
+    )
+
+    proj = build_ex1a_canti2d_eq()
+    proj.validate_references()
+
+    path = tmp_path / "ex1a.osmodel"
+    save_project(proj, path)
+    reloaded = load_project(path)
+    reloaded.validate_references()
+
+    results_dir = Path(tempfile.mkdtemp(prefix="ex1a_eq_"))
+    result = OpenSeesRunner(reloaded).run(reloaded.analyses[1], results_dir=results_dir)
+
+    t = result.time()
+    top = result.node_disp_history(2)
+    ux = top[:, 0]
+    uy = top[:, 1]
+
+    assert len(t) == reloaded.analyses[1].n_steps
+    assert result.dt == pytest.approx(ANALYSIS_DT)
+    assert t[-1] == pytest.approx(ANALYSIS_DT * reloaded.analyses[1].n_steps, abs=ANALYSIS_DT)
+
+    # Dynamic response should oscillate in both directions under the base motion.
+    assert ux.max() > 0.01
+    assert ux.min() < -0.01
+
+    # The elastic column should stay in a physically reasonable range.
+    assert max(abs(ux)) < 0.10 * COLUMN_HEIGHT
+
+    # Gravity remains locked but the input is horizontal, so Uy should stay small.
+    assert max(abs(uy)) < 1.0