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_material_dialog_concrete04.py

@@ -0,0 +1,121 @@
+"""GUI tests for the Concrete04 material form."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import Concrete04  # noqa: E402
+from otko.views.dialogs.material_forms import (  # noqa: E402
+    Concrete04Form,
+    FORM_REGISTRY,
+)
+
+
+@pytest.mark.gui
+def test_concrete04_in_registry(qtbot) -> None:  # type: ignore[no-untyped-def]
+    assert "Concrete04" in FORM_REGISTRY
+    assert FORM_REGISTRY["Concrete04"] is Concrete04Form
+
+
+@pytest.mark.gui
+def test_concrete04_form_constructs(qtbot) -> None:  # type: ignore[no-untyped-def]
+    form = Concrete04Form()
+    qtbot.addWidget(form)
+    assert form.type_label == "Concrete04 — Popovics concrete (optional tension)"
+
+
+@pytest.mark.gui
+def test_concrete04_read_no_tension(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Fill the form and read back a Concrete04 model (no tension branch)."""
+    form = Concrete04Form()
+    qtbot.addWidget(form)
+
+    form._name_edit.setText("C30")
+    form._fpc.setValue(-30e6)
+    form._epsc0.setValue(-0.002)
+    form._epscu.setValue(-0.005)
+    form._Ec.setValue(30e9)
+    form._fct.setValue(0.0)  # 0 → no tension
+    form._et.setValue(0.0)
+    form._material_id = 1
+
+    mat = form.read()
+    assert isinstance(mat, Concrete04)
+    assert mat.id == 1
+    assert mat.name == "C30"
+    assert mat.fpc == pytest.approx(-30e6)
+    assert mat.epsc0 == pytest.approx(-0.002)
+    assert mat.epscu == pytest.approx(-0.005)
+    assert mat.Ec == pytest.approx(30e9)
+    assert mat.fct is None
+    assert mat.et is None
+
+
+@pytest.mark.gui
+def test_concrete04_read_with_tension(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Form with positive fct + et produces the tensile-branch model."""
+    form = Concrete04Form()
+    qtbot.addWidget(form)
+
+    form._fpc.setValue(-30e6)
+    form._epsc0.setValue(-0.002)
+    form._epscu.setValue(-0.005)
+    form._Ec.setValue(30e9)
+    form._fct.setValue(3.0e6)
+    form._et.setValue(1e-4)
+    form._material_id = 2
+
+    mat = form.read()
+    assert isinstance(mat, Concrete04)
+    assert mat.fct == pytest.approx(3.0e6)
+    assert mat.et == pytest.approx(1e-4)
+
+
+@pytest.mark.gui
+def test_concrete04_populate_round_trip(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """populate(mat) + read() reproduces the original model object."""
+    original = Concrete04(
+        id=3,
+        name="C40-Tension",
+        fpc=-40e6,
+        epsc0=-0.0022,
+        epscu=-0.006,
+        Ec=32e9,
+        fct=2.5e6,
+        et=8e-5,
+    )
+
+    form = Concrete04Form()
+    qtbot.addWidget(form)
+    form.populate(original)
+
+    recovered = form.read()
+    assert recovered.id == original.id
+    assert recovered.name == original.name
+    assert recovered.fpc == pytest.approx(original.fpc)
+    assert recovered.epsc0 == pytest.approx(original.epsc0)
+    assert recovered.epscu == pytest.approx(original.epscu)
+    assert recovered.Ec == pytest.approx(original.Ec)
+    assert recovered.fct == pytest.approx(original.fct)
+    assert recovered.et == pytest.approx(original.et)
+
+
+@pytest.mark.gui
+def test_concrete04_accepts_ksi_values(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Form must not clamp US-customary (ksi) values for fpc."""
+    form = Concrete04Form()
+    qtbot.addWidget(form)
+    form._fpc.setValue(-6.0)  # ksi
+    form._Ec.setValue(3600.0)  # ksi
+    assert form._fpc.value() == pytest.approx(-6.0)
+    assert form._Ec.value() == pytest.approx(3600.0)
+
+
+@pytest.mark.gui
+def test_concrete04_spinboxes_use_c_locale(qtbot) -> None:  # type: ignore[no-untyped-def]
+    form = Concrete04Form()
+    qtbot.addWidget(form)
+    assert form._epsc0.locale().decimalPoint() == "."
+    assert form._fpc.locale().decimalPoint() == "."