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_section_forms.py

@@ -0,0 +1,100 @@
+"""Regression: clicking a FiberSection / SectionAggregator row in the
+Section Library must not crash ``form_for`` with KeyError."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import (  # noqa: E402
+    AggregatorDOF,
+    ElasticSection,
+    FiberSection,
+    RectangularPatch,
+    SectionAggregator,
+)
+from otko.views.dialogs.section_forms import (  # noqa: E402
+    FORM_REGISTRY,
+    form_for,
+)
+
+
+def test_fiber_section_has_registered_form() -> None:
+    assert "FiberSection" in FORM_REGISTRY
+
+
+def test_section_aggregator_has_registered_form() -> None:
+    assert "SectionAggregator" in FORM_REGISTRY
+
+
+@pytest.mark.gui
+def test_form_for_elastic_section_returns_editable_form(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = ElasticSection(
+        id=1,
+        name="Rect",
+        E=200e9,
+        A=0.01,
+        Iz=1e-5,
+        Iy=1e-5,
+        G=80e9,
+        J=1e-6,
+    )
+    f = form_for(s)
+    qtbot.addWidget(f)
+    assert f.type_label == "Elastic Section"
+
+
+@pytest.mark.gui
+def test_form_for_fiber_section_returns_summary(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = FiberSection(
+        id=1,
+        name="RC",
+        patches=[
+            RectangularPatch(
+                material_id=1,
+                n_fib_y=4,
+                n_fib_z=4,
+                y_i=-0.1,
+                z_i=-0.15,
+                y_j=0.1,
+                z_j=0.15,
+            )
+        ],
+    )
+    f = form_for(s)
+    qtbot.addWidget(f)
+    assert f.type_label == "Fiber Section"
+    # read() round-trips without crashing.
+    out = f.read(1)
+    assert isinstance(out, FiberSection)
+    assert len(out.patches) == 1
+
+
+@pytest.mark.gui
+def test_form_for_section_aggregator_returns_summary(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SectionAggregator(
+        id=2,
+        name="Agg",
+        section_id=1,
+        pairings=[AggregatorDOF(material_id=3, dof="T")],
+    )
+    f = form_for(s)
+    qtbot.addWidget(f)
+    assert f.type_label == "Section Aggregator"
+
+
+@pytest.mark.gui
+def test_form_for_unknown_type_falls_back(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """An unknown section type must NOT raise — the dialog gets a
+    placeholder form so the whole UI doesn't go down."""
+
+    class _Mystery:
+        type = "NotRegistered"
+        id = 99
+        name = "???"
+
+    f = form_for(_Mystery())
+    qtbot.addWidget(f)
+    # Placeholder form exists; no exception raised.
+    assert f is not None