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_transforms.py

@@ -0,0 +1,152 @@
+"""Unit tests for geometric transform commands."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.commands import (  # noqa: E402
+    AddElementsCommand,
+    AddNodesCommand,
+    MirrorCommand,
+    MoveNodesCommand,
+    ReplicateCommand,
+)
+from otko.core import Node, Steel01, TrussElement  # noqa: E402
+from otko.viewmodels import ProjectViewModel  # noqa: E402
+
+
+def _populated_vm() -> ProjectViewModel:
+    """A small VM with 4 corner nodes + 4 truss elements forming a square."""
+    vm = ProjectViewModel()
+    vm.new_project()
+    vm.apply_command(
+        AddNodesCommand(
+            vm,
+            [
+                Node(id=1, coords=(0, 0, 0), restraint=(True,) * 6),
+                Node(id=2, coords=(1, 0, 0)),
+                Node(id=3, coords=(1, 1, 0)),
+                Node(id=4, coords=(0, 1, 0)),
+            ],
+        )
+    )
+    vm.project.materials.append(Steel01(id=1, Fy=420e6, E0=200e9, b=0.01))
+    vm.apply_command(
+        AddElementsCommand(
+            vm,
+            [
+                TrussElement(id=1, nodes=(1, 2), area=1e-3, material_id=1),
+                TrussElement(id=2, nodes=(2, 3), area=1e-3, material_id=1),
+                TrussElement(id=3, nodes=(3, 4), area=1e-3, material_id=1),
+                TrussElement(id=4, nodes=(4, 1), area=1e-3, material_id=1),
+            ],
+        )
+    )
+    return vm
+
+
+# ──────────────────────────── Move ────────────────────────────
+@pytest.mark.gui
+def test_move_translates_in_place(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    vm.apply_command(MoveNodesCommand(vm, {1, 2, 3, 4}, (10.0, 0.0, 0.0)))
+    assert vm.project.node(1).coords == (10.0, 0.0, 0.0)
+    assert vm.project.node(2).coords == (11.0, 0.0, 0.0)
+    assert len(vm.project.nodes) == 4  # no copies created
+
+
+@pytest.mark.gui
+def test_move_undo_restores_original_coords(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    vm.apply_command(MoveNodesCommand(vm, {1}, (5.0, 0.0, 0.0)))
+    assert vm.project.node(1).coords == (5.0, 0.0, 0.0)
+    vm.undo_stack.undo()
+    assert vm.project.node(1).coords == (0.0, 0.0, 0.0)
+
+
+@pytest.mark.gui
+def test_move_preserves_restraint(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    fix6 = (True,) * 6
+    vm.apply_command(MoveNodesCommand(vm, {1}, (5.0, 0.0, 0.0)))
+    assert vm.project.node(1).restraint == fix6
+
+
+# ──────────────────────────── Replicate ────────────────────────────
+@pytest.mark.gui
+def test_replicate_creates_n_copies(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    vm.apply_command(
+        ReplicateCommand(vm, {1, 2, 3, 4}, {1, 2, 3, 4}, offset=(0, 0, 3.0), n_copies=2)
+    )
+    # Original 4 + 2*4 copies = 12 nodes; original 4 + 2*4 elements = 12 elements
+    assert len(vm.project.nodes) == 12
+    assert len(vm.project.elements) == 12
+    # Replica 1 sits at z=3, replica 2 at z=6.
+    z_values = sorted({n.coords[2] for n in vm.project.nodes})
+    assert z_values == [0.0, 3.0, 6.0]
+
+
+@pytest.mark.gui
+def test_replicate_undo_removes_only_copies(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    vm.apply_command(
+        ReplicateCommand(vm, {1, 2, 3, 4}, {1, 2, 3, 4}, offset=(0, 0, 3.0), n_copies=2)
+    )
+    vm.undo_stack.undo()
+    assert len(vm.project.nodes) == 4
+    assert len(vm.project.elements) == 4
+    # Original ids preserved.
+    assert {n.id for n in vm.project.nodes} == {1, 2, 3, 4}
+
+
+@pytest.mark.gui
+def test_replicate_skips_elements_with_unselected_endpoints(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    # Select only node 1 and 2; elements 1 (1↔2) is fully covered, others aren't.
+    vm.apply_command(ReplicateCommand(vm, {1, 2}, {1, 2, 3, 4}, offset=(0, 0, 3.0), n_copies=1))
+    # 2 new nodes + 1 new element (only element 1 was fully bracketed).
+    assert len(vm.project.nodes) == 6
+    assert len(vm.project.elements) == 5
+
+
+@pytest.mark.gui
+def test_replicate_zero_copies_rejected(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    with pytest.raises(ValueError):
+        ReplicateCommand(vm, {1}, set(), offset=(0, 0, 1), n_copies=0)
+
+
+# ──────────────────────────── Mirror ────────────────────────────
+@pytest.mark.gui
+def test_mirror_yz_flips_x(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    vm.apply_command(MirrorCommand(vm, {1, 2, 3, 4}, {1, 2, 3, 4}, plane="YZ"))
+    assert len(vm.project.nodes) == 8
+    assert len(vm.project.elements) == 8
+    # The mirrored copies should have negative x for original-non-zero x.
+    new_node_ids = {n.id for n in vm.project.nodes} - {1, 2, 3, 4}
+    new_xs = {vm.project.node(nid).coords[0] for nid in new_node_ids}
+    assert -1.0 in new_xs
+
+
+@pytest.mark.gui
+def test_mirror_xy_flips_z(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    # Move the square up first so z=2 (mirroring across z=0 gives -2).
+    vm.apply_command(MoveNodesCommand(vm, {1, 2, 3, 4}, (0, 0, 2.0)))
+    vm.apply_command(MirrorCommand(vm, {1, 2, 3, 4}, {1, 2, 3, 4}, plane="XY"))
+    new_node_ids = {n.id for n in vm.project.nodes} - {1, 2, 3, 4}
+    new_zs = {vm.project.node(nid).coords[2] for nid in new_node_ids}
+    assert new_zs == {-2.0}
+
+
+@pytest.mark.gui
+def test_mirror_undo_removes_copies(qtbot) -> None:  # type: ignore[no-untyped-def]
+    vm = _populated_vm()
+    vm.apply_command(MirrorCommand(vm, {1, 2, 3, 4}, {1, 2, 3, 4}, plane="YZ"))
+    vm.undo_stack.undo()
+    assert len(vm.project.nodes) == 4
+    assert len(vm.project.elements) == 4