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_tree_multi_select.py

@@ -0,0 +1,61 @@
+"""Model Tree supports Ctrl+click multi-selection — regression guard."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from PySide6.QtCore import Qt  # noqa: E402
+from PySide6.QtWidgets import QTreeWidget  # noqa: E402
+
+from otko.core import Node  # noqa: E402
+
+
+@pytest.mark.gui
+def test_tree_is_in_extended_selection_mode(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Default QTreeWidget is single-select; we need ExtendedSelection."""
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    assert mw._tree.selectionMode() == QTreeWidget.SelectionMode.ExtendedSelection
+
+
+@pytest.mark.gui
+def test_tree_multi_select_syncs_canvas(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Selecting two node rows in the tree must populate the canvas
+    selection with both ids — the precondition for the Zero-Length
+    Section dialog (exactly 2 joints)."""
+    from otko.views.main_window import MainWindow
+
+    mw = MainWindow()
+    qtbot.addWidget(mw)
+    mw._vm.new_project(ndm=2, ndf=3)
+    mw._vm.project.nodes.extend(
+        [
+            Node(id=1, coords=(0, 0, 0)),
+            Node(id=2, coords=(0, 0, 0)),
+            Node(id=3, coords=(1, 0, 0)),
+        ]
+    )
+    mw._refresh_tree(mw._vm.project)
+
+    nodes_cat = mw._tree_categories["Nodes"]
+    nodes_cat.setExpanded(True)
+    # Find the three child items and select rows 0 + 1.
+    items = [nodes_cat.child(i) for i in range(nodes_cat.childCount())]
+    assert len(items) == 3
+    items[0].setSelected(True)
+    items[1].setSelected(True)
+
+    assert mw._canvas.selection.nodes == frozenset({1, 2})
+    assert mw._canvas.selection.elements == frozenset()
+
+    # Adding a third selection (row 2) keeps all three.
+    items[2].setSelected(True)
+    assert mw._canvas.selection.nodes == frozenset({1, 2, 3})
+
+    # Deselecting row 0 leaves rows 1 + 2.
+    items[0].setSelected(False)
+    assert mw._canvas.selection.nodes == frozenset({2, 3})