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_moment_curvature_example.py

@@ -0,0 +1,56 @@
+"""Round-trip + physics check on the shipped moment-curvature example."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("openseespy")
+
+from otko.services import load_project  # noqa: E402
+from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def test_moment_curvature_example_round_trips_and_converges(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    """build_moment_curvature() → save → load → run → expected shape."""
+    from examples.moment_curvature import (
+        build_moment_curvature,
+        COL_DEPTH,
+        COVER,
+        E_STEEL,
+        FY,
+        MU,
+        NUM_INCR,
+    )
+
+    proj = build_moment_curvature()
+    proj.validate_references()
+
+    # Save + reload — catches schema drift.
+    from otko.services import save_project
+
+    path = tmp_path / "mk.osmodel"
+    save_project(proj, path)
+    reloaded = load_project(path)
+    reloaded.validate_references()
+    assert reloaded.meta.units.value.startswith("US (in,")
+
+    result = OpenSeesRunner(reloaded).run(reloaded.analyses[0])
+
+    # Yield curvature estimate.
+    d = COL_DEPTH - COVER
+    ky = (FY / E_STEEL) / (0.7 * d)
+
+    # At least half of the NUM_INCR pushover steps converged.
+    assert (
+        len(result.control_disp) > NUM_INCR * 0.5
+    ), "Pushover bailed out prematurely — check Concrete01 softening"
+    # Reached the mu * Ky target.
+    assert result.control_disp[-1] == pytest.approx(MU * ky, rel=1e-2)
+    # Moment at yield curvature is plausible: > 3 kip·in and < 10 kip·in per rebar
+    # → for 8 bars total, the section moment capacity is roughly O(3000-6000) kip·in.
+    peak_moment = max(abs(m) for m in result.base_shear)
+    assert 2000 < peak_moment < 10000, (
+        f"Peak moment {peak_moment:.1f} kip·in is outside the " "expected RC section range"
+    )