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/core/materials/test_concrete04.py

@@ -0,0 +1,147 @@
+"""Unit tests for Concrete04 (Popovics concrete)."""
+
+from __future__ import annotations
+
+import pytest
+from pydantic import TypeAdapter, ValidationError
+
+from otko.core import Concrete04, Material
+
+material_adapter: TypeAdapter[Material] = TypeAdapter(Material)
+
+
+class TestConcrete04Instantiation:
+    def test_no_tension_construct(self) -> None:
+        m = Concrete04(id=1, name="C30-NoTension", fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9)
+        assert m.type == "Concrete04"
+        assert m.fpc == -30e6
+        assert m.epsc0 == -0.002
+        assert m.epscu == -0.005
+        assert m.Ec == 30e9
+        assert m.fct is None
+        assert m.et is None
+        assert m.beta is None
+
+    def test_with_tension_construct(self) -> None:
+        m = Concrete04(
+            id=2,
+            name="C30-Tension",
+            fpc=-30e6,
+            epsc0=-0.002,
+            epscu=-0.005,
+            Ec=30e9,
+            fct=3.0e6,
+            et=1e-4,
+        )
+        assert m.fct == 3.0e6
+        assert m.et == 1e-4
+        assert m.beta is None
+
+    def test_with_beta_construct(self) -> None:
+        m = Concrete04(
+            id=3,
+            name="C30-Cyclic",
+            fpc=-30e6,
+            epsc0=-0.002,
+            epscu=-0.005,
+            Ec=30e9,
+            fct=3.0e6,
+            et=1e-4,
+            beta=0.1,
+        )
+        assert m.beta == pytest.approx(0.1)
+
+    def test_defaults_are_no_tension(self) -> None:
+        m = Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9)
+        assert m.fct is None and m.et is None and m.beta is None
+
+
+class TestConcrete04SignConventions:
+    def test_fpc_must_be_negative(self) -> None:
+        with pytest.raises(ValidationError):
+            Concrete04(id=1, fpc=30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9)
+
+    def test_epsc0_must_be_negative(self) -> None:
+        with pytest.raises(ValidationError):
+            Concrete04(id=1, fpc=-30e6, epsc0=0.002, epscu=-0.005, Ec=30e9)
+
+    def test_epscu_must_be_negative(self) -> None:
+        with pytest.raises(ValidationError):
+            Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=0.005, Ec=30e9)
+
+    def test_ec_must_be_positive(self) -> None:
+        with pytest.raises(ValidationError):
+            Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=-30e9)
+
+    def test_fct_must_be_positive(self) -> None:
+        with pytest.raises(ValidationError):
+            Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9, fct=-1.0, et=1e-4)
+
+
+class TestConcrete04TensileParamConsistency:
+    def test_fct_without_et_raises(self) -> None:
+        with pytest.raises(ValidationError, match="et is required"):
+            Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9, fct=3e6)
+
+    def test_et_without_fct_raises(self) -> None:
+        with pytest.raises(ValidationError, match="fct is required"):
+            Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9, et=1e-4)
+
+    def test_beta_without_fct_et_raises(self) -> None:
+        with pytest.raises(ValidationError, match="beta requires"):
+            Concrete04(id=1, fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9, beta=0.1)
+
+    def test_beta_bounds(self) -> None:
+        with pytest.raises(ValidationError):
+            Concrete04(
+                id=1,
+                fpc=-30e6,
+                epsc0=-0.002,
+                epscu=-0.005,
+                Ec=30e9,
+                fct=3e6,
+                et=1e-4,
+                beta=1.5,
+            )
+
+
+class TestConcrete04JsonRoundTrip:
+    def test_no_tension_round_trip(self) -> None:
+        original = Concrete04(id=1, name="C30", fpc=-30e6, epsc0=-0.002, epscu=-0.005, Ec=30e9)
+        payload = material_adapter.dump_python(original, mode="json", by_alias=True)
+        assert payload["type"] == "Concrete04"
+        restored = material_adapter.validate_python(payload)
+        assert type(restored) is Concrete04
+        assert restored.id == original.id
+        assert restored.fpc == original.fpc
+        assert restored.fct is None
+
+    def test_with_tension_round_trip(self) -> None:
+        original = Concrete04(
+            id=2,
+            name="C30-T",
+            fpc=-30e6,
+            epsc0=-0.002,
+            epscu=-0.005,
+            Ec=30e9,
+            fct=3.0e6,
+            et=1e-4,
+        )
+        payload = material_adapter.dump_python(original, mode="json", by_alias=True)
+        restored = material_adapter.validate_python(payload)
+        assert type(restored) is Concrete04
+        assert restored.fct == pytest.approx(3.0e6)
+        assert restored.et == pytest.approx(1e-4)
+
+    def test_old_osmodel_without_concrete04_loads_cleanly(self) -> None:
+        """An osmodel payload that doesn't mention Concrete04 is unaffected."""
+        from pathlib import Path
+        from otko.services import load_project
+
+        osmodel = Path(__file__).parents[4] / "examples" / "cantilever.osmodel"
+        if not osmodel.exists():
+            pytest.skip("cantilever.osmodel not present")
+        proj = load_project(osmodel)
+        proj.validate_references()
+        # No Concrete04 in a cantilever; confirm it still round-trips cleanly.
+        assert all(m.type != "Concrete04" for m in proj.materials)