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/unit/test_section_shape.py

@@ -0,0 +1,98 @@
+"""Tests for the optional, cosmetic section-shape hint on ``ElasticSection``.
+
+The shape (pipe/angle/rect) is a *drawing hint* only — it records the true
+cross-section geometry so a viewer can extrude a tube as a tube and an angle as
+an L, instead of back-solving an equivalent box from A/Iz. It is never emitted to
+OpenSees and never read by the runner, so attaching one must never perturb the
+stiffness fields the analysis actually uses.
+"""
+
+from __future__ import annotations
+
+import pytest
+from pydantic import ValidationError
+
+from otko.core import (
+    AngleShape,
+    ElasticSection,
+    PipeShape,
+    RectShape,
+)
+
+
+def _elastic(**over: object) -> ElasticSection:
+    base: dict[str, object] = dict(
+        id=1, name="S", E=210e6, A=0.017279, Iz=1.642e-4, Iy=1.642e-4, G=27.7e6, J=3.284e-4
+    )
+    base.update(over)
+    return ElasticSection(**base)  # type: ignore[arg-type]
+
+
+# ── default + back-compat ─────────────────────────────────────────
+def test_shape_defaults_to_none() -> None:
+    """An ElasticSection built without a shape has none — older models load fine."""
+    assert _elastic().shape is None
+
+
+def test_legacy_json_without_shape_still_validates() -> None:
+    """A serialised section that predates the field (no ``shape`` key) round-trips."""
+    sec = ElasticSection.model_validate(
+        {"id": 1, "name": "S", "type": "ElasticSection", "E": 1.0, "A": 1.0, "Iz": 1.0}
+    )
+    assert sec.shape is None
+
+
+# ── the three shape kinds ─────────────────────────────────────────
+def test_pipe_shape_roundtrips() -> None:
+    sec = _elastic(shape=PipeShape(od=0.295, t=0.020))
+    restored = ElasticSection.model_validate(sec.model_dump(mode="json"))
+    assert isinstance(restored.shape, PipeShape)
+    assert restored.shape.od == pytest.approx(0.295)
+    assert restored.shape.t == pytest.approx(0.020)
+
+
+def test_angle_shape_roundtrips() -> None:
+    sec = _elastic(shape=AngleShape(d=0.08, b=0.08, t=0.008))
+    restored = ElasticSection.model_validate(sec.model_dump(mode="json"))
+    assert isinstance(restored.shape, AngleShape)
+    assert restored.shape.d == pytest.approx(0.08)
+    assert restored.shape.b == pytest.approx(0.08)
+    assert restored.shape.t == pytest.approx(0.008)
+
+
+def test_rect_shape_roundtrips() -> None:
+    sec = _elastic(shape=RectShape(d=0.13, b=0.016))
+    restored = ElasticSection.model_validate(sec.model_dump(mode="json"))
+    assert isinstance(restored.shape, RectShape)
+    assert (restored.shape.d, restored.shape.b) == pytest.approx((0.13, 0.016))
+
+
+def test_shape_accepts_a_plain_dict_via_the_kind_discriminator() -> None:
+    """A builder may pass a dict; the discriminated union selects the right class."""
+    sec = _elastic(shape={"kind": "pipe", "od": 0.37, "t": 0.03})
+    assert isinstance(sec.shape, PipeShape)
+
+
+# ── the hint never touches the analysis fields ────────────────────
+def test_shape_does_not_change_the_stiffness_fields() -> None:
+    """Attaching a shape leaves A/Iz/Iy/J byte-identical — same emission to OpenSees."""
+    plain = _elastic()
+    shaped = _elastic(shape=PipeShape(od=0.295, t=0.020))
+    for f in ("E", "A", "Iz", "Iy", "G", "J"):
+        assert getattr(plain, f) == getattr(shaped, f)
+    # The only difference in the serialised form is the added shape key.
+    pj, sj = plain.model_dump(mode="json"), shaped.model_dump(mode="json")
+    assert sj.pop("shape") == {"kind": "pipe", "od": 0.295, "t": 0.020}
+    assert pj.pop("shape") is None
+    assert pj == sj
+
+
+# ── validation guards ─────────────────────────────────────────────
+def test_unknown_shape_kind_is_rejected() -> None:
+    with pytest.raises(ValidationError):
+        _elastic(shape={"kind": "ibeam", "d": 0.1, "b": 0.1})
+
+
+def test_shape_dims_must_be_positive() -> None:
+    with pytest.raises(ValidationError):
+        _elastic(shape={"kind": "pipe", "od": 0.0, "t": 0.01})