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_node.py

@@ -0,0 +1,53 @@
+"""Unit tests for the Node entity."""
+
+from __future__ import annotations
+
+import pytest
+from pydantic import ValidationError
+
+from otko.core import Node
+
+
+class TestNodeConstruction:
+    def test_minimum_required_fields(self) -> None:
+        n = Node(id=1, coords=(0.0, 0.0, 0.0))
+        assert n.id == 1
+        assert n.coords == (0.0, 0.0, 0.0)
+        assert n.mass == (0.0,) * 6
+        assert n.restraint == (False,) * 6
+        assert n.is_free
+        assert not n.is_restrained
+
+    def test_with_name_and_mass(self) -> None:
+        n = Node(
+            id=42, name="A1", coords=(1.0, 2.0, 3.0), mass=(100.0, 100.0, 100.0, 0.0, 0.0, 0.0)
+        )
+        assert n.name == "A1"
+        assert n.mass[0] == 100.0
+
+    def test_pin_restraint(self) -> None:
+        pin = Node(id=1, coords=(0, 0, 0), restraint=(True, True, True, False, False, False))
+        assert pin.is_restrained
+        assert sum(pin.restraint) == 3
+
+
+class TestNodeValidation:
+    def test_id_must_be_positive(self) -> None:
+        with pytest.raises(ValidationError):
+            Node(id=0, coords=(0, 0, 0))
+        with pytest.raises(ValidationError):
+            Node(id=-1, coords=(0, 0, 0))
+
+    def test_coords_must_be_three_floats(self) -> None:
+        with pytest.raises(ValidationError):
+            Node(id=1, coords=(0.0, 0.0))  # type: ignore[arg-type]
+        with pytest.raises(ValidationError):
+            Node(id=1, coords=(0.0, 0.0, 0.0, 0.0))  # type: ignore[arg-type]
+
+    def test_mass_must_be_six_components(self) -> None:
+        with pytest.raises(ValidationError):
+            Node(id=1, coords=(0, 0, 0), mass=(1.0, 2.0, 3.0))  # type: ignore[arg-type]
+
+    def test_extra_fields_rejected(self) -> None:
+        with pytest.raises(ValidationError):
+            Node(id=1, coords=(0, 0, 0), unknown="oops")  # type: ignore[call-arg]