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_units_status_bar.py

@@ -0,0 +1,57 @@
+"""Status-bar unit picker syncs with Project.meta.units."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import UnitSystem  # noqa: E402
+
+
+@pytest.mark.gui
+def test_status_bar_combo_lists_all_unit_systems(qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    assert mw._units_combo.count() == len(list(UnitSystem))
+
+
+@pytest.mark.gui
+def test_status_bar_combo_reflects_project_units(qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    mw._vm.new_project()
+    mw._vm.project.meta.units = UnitSystem.US_IN_KIP
+    mw._sync_units_combo()
+    assert mw._units_combo.currentData() == UnitSystem.US_IN_KIP
+
+
+@pytest.mark.gui
+def test_status_bar_combo_write_updates_project(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Changing the combo writes through to project.meta.units."""
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    mw._vm.new_project()
+    # Default is SI_M_N — switch to US_IN_KIP via the combo.
+    target_idx = mw._units_combo.findData(UnitSystem.US_IN_KIP)
+    mw._units_combo.setCurrentIndex(target_idx)
+    assert mw._vm.project.meta.units == UnitSystem.US_IN_KIP
+    assert mw._vm.is_dirty
+
+
+@pytest.mark.gui
+def test_status_bar_combo_no_project_noop(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Before a project is loaded, changing the combo is a no-op."""
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    # No crash even with no project.
+    mw._units_combo.setCurrentIndex(2)
+    assert mw._vm.project is None