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_selection.py

@@ -0,0 +1,70 @@
+"""Unit tests for the SelectionState QObject."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.views.canvas3d import SelectionState  # noqa: E402
+
+
+@pytest.mark.gui
+def test_initial_state_is_empty(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    assert s.is_empty
+    assert s.nodes == frozenset()
+    assert s.elements == frozenset()
+
+
+@pytest.mark.gui
+def test_select_node_emits_signal(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    with qtbot.waitSignal(s.selectionChanged, timeout=500) as blocker:
+        s.select_node(7)
+    assert blocker.args[0] == frozenset({7})
+    assert blocker.args[1] == frozenset()
+
+
+@pytest.mark.gui
+def test_replacing_selection_clears_other_kind(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    s.select_node(1)
+    s.select_element(5)
+    assert s.nodes == frozenset()
+    assert s.elements == frozenset({5})
+
+
+@pytest.mark.gui
+def test_additive_selection_keeps_both(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    s.select_node(1)
+    s.select_node(2, additive=True)
+    assert s.nodes == frozenset({1, 2})
+
+
+@pytest.mark.gui
+def test_toggle_node(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    s.toggle_node(3)
+    assert 3 in s.nodes
+    s.toggle_node(3)
+    assert 3 not in s.nodes
+
+
+@pytest.mark.gui
+def test_clear_no_op_when_empty_does_not_emit(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    received: list[tuple] = []
+    s.selectionChanged.connect(lambda n, e: received.append((n, e)))
+    s.clear()
+    assert received == []
+
+
+@pytest.mark.gui
+def test_clear_emits_when_non_empty(qtbot) -> None:  # type: ignore[no-untyped-def]
+    s = SelectionState()
+    s.select_node(1)
+    with qtbot.waitSignal(s.selectionChanged, timeout=500):
+        s.clear()
+    assert s.is_empty