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_elements.py

@@ -0,0 +1,63 @@
+"""Unit tests for element types."""
+
+from __future__ import annotations
+
+import pytest
+from pydantic import TypeAdapter, ValidationError
+
+from otko.core import (
+    Element,
+    ElasticBeamColumn,
+    ForceBeamColumn,
+    TrussElement,
+    ZeroLengthElement,
+)
+
+element_adapter: TypeAdapter[Element] = TypeAdapter(Element)
+
+
+def test_truss_construct() -> None:
+    t = TrussElement(id=1, nodes=(1, 2), area=0.01, material_id=1)
+    assert t.type == "Truss"
+    assert t.nodes == (1, 2)
+
+
+def test_truss_requires_two_nodes() -> None:
+    with pytest.raises(ValidationError):
+        TrussElement(id=1, nodes=(1,), area=0.01, material_id=1)  # type: ignore[arg-type]
+    with pytest.raises(ValidationError):
+        TrussElement(id=1, nodes=(1, 2, 3), area=0.01, material_id=1)  # type: ignore[arg-type]
+
+
+def test_truss_area_must_be_positive() -> None:
+    with pytest.raises(ValidationError):
+        TrussElement(id=1, nodes=(1, 2), area=-0.01, material_id=1)
+
+
+def test_elastic_beam_default_geom_transf() -> None:
+    e = ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1)
+    assert e.geom_transf == "Linear"
+
+
+def test_force_beam_integration_points_bounds() -> None:
+    ForceBeamColumn(id=1, nodes=(1, 2), section_id=1, integration_points=5)
+    with pytest.raises(ValidationError):
+        ForceBeamColumn(id=1, nodes=(1, 2), section_id=1, integration_points=1)
+    with pytest.raises(ValidationError):
+        ForceBeamColumn(id=1, nodes=(1, 2), section_id=1, integration_points=11)
+
+
+def test_zero_length_dofs_match_materials() -> None:
+    z = ZeroLengthElement(id=1, nodes=(1, 2), material_ids=(1, 2), dofs=(1, 2))
+    assert z.dofs == (1, 2)
+
+
+def test_element_union_round_trip() -> None:
+    for original in [
+        TrussElement(id=1, nodes=(1, 2), area=0.01, material_id=1),
+        ElasticBeamColumn(id=2, nodes=(1, 2), section_id=1, geom_transf="PDelta"),
+    ]:
+        payload = element_adapter.dump_python(original, mode="json")
+        restored = element_adapter.validate_python(payload)
+        assert type(restored) is type(original)
+        assert restored.id == original.id