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_main_window.py

@@ -0,0 +1,41 @@
+"""Smoke test for the MainWindow.
+
+Marked as ``gui`` so it can be deselected on environments without a display.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("pyvistaqt")
+
+
+@pytest.mark.gui
+def test_main_window_opens(qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    window = MainWindow()
+    qtbot.addWidget(window)
+    window.show()
+    assert window.isVisible()
+    assert window.windowTitle() == "OTKO"
+
+
+@pytest.mark.gui
+def test_close_dirty_hidden_window_does_not_block(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Closing a dirty but hidden window must not raise a modal prompt.
+
+    Qt closes hidden widgets during teardown (as ``pytest-qt`` does); a
+    modal "unsaved changes" dialog there would block forever with no user
+    to answer it. The prompt is therefore gated on visibility.
+    """
+    from otko.views.main_window import MainWindow
+
+    window = MainWindow()
+    qtbot.addWidget(window)
+    window._vm.new_project()
+    window._vm.mark_dirty()
+    assert not window.isVisible()
+    assert window.close() is True
+    assert window._vm.is_dirty  # unchanged: the prompt was skipped