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/gui/test_assign_zls.py

@@ -0,0 +1,62 @@
+"""Unit tests for the Assign Zero-Length Section dialog."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import (  # noqa: E402
+    ElasticSection,
+    Node,
+    Project,
+)
+from otko.views.dialogs.assign_zls import (  # noqa: E402
+    AssignZeroLengthSectionDialog,
+)
+
+
+def _project_with_section() -> Project:
+    return Project(
+        ndm=2,
+        ndf=3,
+        nodes=[
+            Node(id=1, coords=(0, 0, 0)),
+            Node(id=2, coords=(0, 0, 0)),
+        ],
+        sections=[
+            ElasticSection(
+                id=3,
+                name="MK",
+                E=30000,
+                A=200,
+                Iz=6667,
+                Iy=6667,
+                G=12000,
+                J=100,
+            )
+        ],
+    )
+
+
+@pytest.mark.gui
+def test_dialog_lists_project_sections(qtbot) -> None:  # type: ignore[no-untyped-def]
+    p = _project_with_section()
+    dlg = AssignZeroLengthSectionDialog(p, (1, 2))
+    qtbot.addWidget(dlg)
+    assert dlg._section_cb.count() == 1
+    assert dlg._section_cb.itemData(0) == 3
+    assert dlg.section_id() == 3
+
+
+@pytest.mark.gui
+def test_dialog_disabled_when_no_sections(qtbot) -> None:  # type: ignore[no-untyped-def]
+    p = Project(
+        ndm=2,
+        ndf=3,
+        nodes=[Node(id=1, coords=(0, 0, 0)), Node(id=2, coords=(0, 0, 0))],
+    )
+    dlg = AssignZeroLengthSectionDialog(p, (1, 2))
+    qtbot.addWidget(dlg)
+    assert not dlg._section_cb.isEnabled()
+    assert dlg.section_id() is None