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_zerolength_section.py

@@ -0,0 +1,64 @@
+"""Unit tests for ZeroLengthSectionElement model + serialization."""
+
+from __future__ import annotations
+
+import pytest
+
+from otko.core import (
+    ElasticSection,
+    Node,
+    Project,
+    ZeroLengthSectionElement,
+)
+from otko.services import load_project, save_project
+
+
+def test_zero_length_section_schema_defaults() -> None:
+    el = ZeroLengthSectionElement(id=1, nodes=(1, 2), section_id=5)
+    assert el.type == "ZeroLengthSection"
+    assert el.nodes == (1, 2)
+    assert el.section_id == 5
+
+
+def test_zero_length_section_rejects_extra_fields() -> None:
+    with pytest.raises(Exception):
+        ZeroLengthSectionElement(
+            id=1,
+            nodes=(1, 2),
+            section_id=1,
+            bogus="not allowed",  # type: ignore[call-arg]
+        )
+
+
+def test_zero_length_section_round_trips(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    """A project containing a zeroLengthSection must save+load unchanged."""
+    p = Project(
+        ndm=2,
+        ndf=3,
+        nodes=[
+            Node(id=1, coords=(0, 0, 0), restraint=(True, True, False, False, False, True)),
+            Node(id=2, coords=(0, 0, 0), restraint=(False, True, False, False, False, False)),
+        ],
+        sections=[
+            ElasticSection(
+                id=1,
+                name="Box",
+                E=200e9,
+                A=0.01,
+                Iz=1e-5,
+                Iy=1e-5,
+                G=80e9,
+                J=1e-6,
+            )
+        ],
+        elements=[
+            ZeroLengthSectionElement(id=10, nodes=(1, 2), section_id=1),
+        ],
+    )
+    path = tmp_path / "mk.osmodel"
+    save_project(p, path)
+    r = load_project(path)
+    assert len(r.elements) == 1
+    assert isinstance(r.elements[0], ZeroLengthSectionElement)
+    assert r.elements[0].nodes == (1, 2)
+    assert r.elements[0].section_id == 1