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_rc_frame_earthquake.py

@@ -0,0 +1,56 @@
+"""Integration test: RC Frame Earthquake example (OpenSees Ex 3.3)."""
+
+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_rc_frame_earthquake_runs_and_has_oscillatory_response(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    """Synthetic ground motion produces bounded, oscillatory response."""
+    from examples.rc_frame_earthquake import (
+        build_rc_frame_earthquake,
+        DT,
+        N_PTS,
+    )
+
+    proj = build_rc_frame_earthquake()
+    proj.validate_references()
+
+    osmodel = tmp_path / "eq.osmodel"
+    save_project(proj, osmodel)
+    reloaded = load_project(osmodel)
+    reloaded.validate_references()
+
+    results_dir = Path(tempfile.mkdtemp(prefix="eq_"))
+    result = OpenSeesRunner(reloaded).run(reloaded.analyses[0], results_dir=results_dir)
+
+    # Simulation covers most of the 4-second record (ModifiedNewton
+    # fallback may trim a few steps at stiffness jumps; we allow that).
+    assert result.n_steps >= int(
+        0.9 * N_PTS
+    ), f"Only {result.n_steps}/{N_PTS} steps — fallback didn't recover"
+
+    # Node 3 Ux history: bounded, non-trivial, some positive AND some
+    # negative (oscillation confirms the base excitation actually
+    # propagated through the mass + damping chain, not a one-shot push).
+    h3 = result.node_disp_history(3)
+    ux = h3[:, 0]
+    assert max(ux) > 0.05, f"max Ux {max(ux):.4f} too small — did excitation apply?"
+    assert min(ux) < -0.05, f"min Ux {min(ux):.4f} — no negative excursion"
+    # Drift stays reasonable (< 10% of column height).
+    assert max(abs(ux)) < 14.4, f"|Ux|_max = {max(abs(ux)):.2f} exceeds 10% drift"
+
+    # Uy on the top nodes — small compared to Ux (gravity holds, base
+    # excitation is horizontal).
+    uy = h3[:, 1]
+    assert max(abs(uy)) < 1.0, f"max |Uy| = {max(abs(uy)):.4f} in too large"