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_basic_truss.py

@@ -0,0 +1,54 @@
+"""OpenSees integration test for the Basic Truss example.
+
+Verifies the Example 1 (3-bar truss) model builds, runs, and produces
+a physically reasonable tip deflection. This is the first of the
+OpenSees Examples Manual regression tests.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("openseespy")
+
+from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+
+pytestmark = pytest.mark.slow
+
+
+def test_basic_truss_matches_opensees_tcl_reference() -> None:
+    """Crown displacement must match the OpenSees Tcl Example-1 reference.
+
+    The Tcl model (kip-in) reports node 4 disp = (+0.5301, -0.1779) in.
+    Our SI model converts to that: 0.5301" = 0.01346 m,
+    0.1779" = 0.004518 m. Tolerance is tight because the SI model is
+    a linear rescale of the Tcl model — any deviation would flag a
+    real solver/translation issue.
+    """
+    from examples.basic_truss import build_basic_truss, IN_TO_M
+
+    proj = build_basic_truss()
+    runner = OpenSeesRunner(proj)
+    result = runner.run(proj.analyses[0])
+
+    assert 4 in result.node_disp
+    ux, uy = result.node_disp[4][-1]
+    ux_in = ux / IN_TO_M
+    uy_in = uy / IN_TO_M
+    assert ux_in == pytest.approx(+0.5301, abs=1e-3), f"Ux = {ux_in} in"
+    assert uy_in == pytest.approx(-0.1779, abs=1e-3), f"Uy = {uy_in} in"
+
+    # All three truss bars must have recorded forces.
+    assert set(result.element_forces.keys()) == {1, 2, 3}
+
+
+def test_basic_truss_round_trips(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    """The example project must survive save/load without any information loss."""
+    from examples.basic_truss import build_basic_truss
+    from otko.services import load_project, save_project
+
+    p = build_basic_truss()
+    path = tmp_path / "basic_truss.osmodel"
+    save_project(p, path)
+    r = load_project(path)
+    assert r.model_dump(by_alias=True) == p.model_dump(by_alias=True)