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_pushover.py

@@ -0,0 +1,72 @@
+"""Integration test for RC Frame Pushover example (OpenSees Ex 3.2)."""
+
+from __future__ import annotations
+
+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_pushover_reaches_target_with_fallback(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    """The 15-in pushover requires the ModifiedNewton convergence
+    fallback to finish; without it the Newton solver stalls in the
+    softening regime. This test asserts the full curve is produced
+    AND shows expected nonlinear shape.
+    """
+    from examples.rc_frame_pushover import (
+        build_rc_frame_pushover,
+        D_STEP,
+        D_TARGET,
+    )
+    from otko.core import PushoverCase
+
+    proj = build_rc_frame_pushover()
+    proj.validate_references()
+
+    path = tmp_path / "rc_push.osmodel"
+    save_project(proj, path)
+    reloaded = load_project(path)
+    reloaded.validate_references()
+
+    # Pushover is one of several cases now (preload + pushover) — pick
+    # by type instead of index.
+    push_case = next(c for c in reloaded.analyses if isinstance(c, PushoverCase))
+    result = OpenSeesRunner(reloaded).run(push_case)
+
+    expected_pts = int(D_TARGET / D_STEP) + 1  # 151 including step 0
+    assert len(result.control_disp) == expected_pts, (
+        f"Got {len(result.control_disp)} points, expected {expected_pts} — "
+        "ModifiedNewton fallback probably didn't kick in."
+    )
+    # Reached the target displacement.
+    assert result.control_disp[-1] == pytest.approx(D_TARGET, rel=1e-3)
+
+    # Curve shape: monotonic climb followed by near-plateau (yielding).
+    # The elastic slope should exceed the post-yield slope by > 4x.
+    early_slope = (result.base_shear[10] - result.base_shear[0]) / (
+        result.control_disp[10] - result.control_disp[0]
+    )
+    late_slope = (result.base_shear[-1] - result.base_shear[-20]) / (
+        result.control_disp[-1] - result.control_disp[-20]
+    )
+    assert early_slope > 4 * late_slope, (
+        f"Early slope {early_slope:.2f} not >> late slope {late_slope:.2f} — "
+        "no yielding visible in the curve."
+    )
+
+    # Peak base shear in a reasonable band for this frame — 150-250 kip.
+    peak = max(abs(result.base_shear))
+    assert 100.0 < peak < 300.0, f"Peak base shear {peak:.1f} kip out of band"
+
+    # Gravity preload stayed applied — column axial force at the start
+    # of the pushover (step 1) should be close to P = 180 kip.
+    step1_col1 = result.element_forces[1][1]
+    axial_step1 = abs(step1_col1[0])
+    assert (
+        100.0 < axial_step1 < 260.0
+    ), f"Col 1 axial at step 1 = {axial_step1:.1f} kip — gravity preload lost?"