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

examples/eigen_two_storey_shear_frame.py

@@ -0,0 +1,117 @@
+"""Eigen analysis of a two-storey shear frame.
+
+OpenSees Wiki tutorial:
+https://opensees.berkeley.edu/wiki/index.php?title=Eigen_analysis_of_a_two-story_shear_frame
+
+Idealized two-storey shear frame from Chopra Example 10.4:
+- 2D frame, ndm=2 / ndf=3
+- beams modeled as flexurally rigid (very large Iz)
+- floor nodes tied with equalDOF in Uy and Rz
+- lumped masses only in Ux
+- modal analysis for the first two modes
+
+Run from the repository root:
+
+    python examples/eigen_two_storey_shear_frame.py
+
+Produces ``examples/eigen_two_storey_shear_frame.osmodel``.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+import sys
+
+if __package__ is None or __package__ == "":
+    sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
+
+from otko.core import (  # noqa: E402
+    ElasticBeamColumn,
+    ElasticSection,
+    EqualDOFConstraint,
+    ModalCase,
+    Node,
+    Project,
+    ProjectMeta,
+    UnitSystem,
+)
+from otko.services import load_project, save_project  # noqa: E402
+
+
+M = 100.0 / 386.0
+NUM_MODES = 2
+
+AC = 63.41
+IC = 320.0
+E = 30000.0
+IB = 10e12
+AB = 63.41
+
+L = 288.0
+H = 144.0
+
+REFERENCE_TCL = Path(__file__).resolve().parent / "data" / "EigenAnal_twoStoreyShearFrame7.tcl.txt"
+
+
+def build_eigen_two_storey_shear_frame() -> Project:
+    return Project(
+        meta=ProjectMeta(
+            name="Eigen - Two-Storey Shear Frame",
+            author="OpenSees Wiki / Chopra Example 10.4",
+            description=(
+                "Two-storey shear frame with equalDOF floor constraints and "
+                "two-mode eigenvalue analysis."
+            ),
+            units=UnitSystem.US_IN_KIP,
+        ),
+        ndm=2,
+        ndf=3,
+        nodes=[
+            Node(id=1, name="N1", coords=(0.0, 0.0, 0.0), restraint=(True, True, False, False, False, True)),
+            Node(id=2, name="N2", coords=(L, 0.0, 0.0), restraint=(True, True, False, False, False, True)),
+            Node(id=3, name="N3", coords=(0.0, H, 0.0), mass=(M, 0.0, 0.0, 0.0, 0.0, 0.0)),
+            Node(id=4, name="N4", coords=(L, H, 0.0), mass=(M, 0.0, 0.0, 0.0, 0.0, 0.0)),
+            Node(id=5, name="N5", coords=(0.0, 2.0 * H, 0.0), mass=(M / 2.0, 0.0, 0.0, 0.0, 0.0, 0.0)),
+            Node(id=6, name="N6", coords=(L, 2.0 * H, 0.0), mass=(M / 2.0, 0.0, 0.0, 0.0, 0.0, 0.0)),
+        ],
+        mp_constraints=[
+            EqualDOFConstraint(retained_node=3, constrained_node=4, dofs=(2, 3)),
+            EqualDOFConstraint(retained_node=5, constrained_node=6, dofs=(2, 3)),
+        ],
+        sections=[
+            ElasticSection(id=1, name="Column-L1", E=E, A=AC, Iz=2.0 * IC),
+            ElasticSection(id=2, name="Column-L2", E=E, A=AC, Iz=IC),
+            ElasticSection(id=3, name="Beam-Rigid", E=E, A=AB, Iz=IB),
+        ],
+        elements=[
+            ElasticBeamColumn(id=1, name="C1", nodes=(1, 3), section_id=1, geom_transf="Linear"),
+            ElasticBeamColumn(id=2, name="C2", nodes=(3, 5), section_id=2, geom_transf="Linear"),
+            ElasticBeamColumn(id=3, name="C3", nodes=(2, 4), section_id=1, geom_transf="Linear"),
+            ElasticBeamColumn(id=4, name="C4", nodes=(4, 6), section_id=2, geom_transf="Linear"),
+            ElasticBeamColumn(id=5, name="B1", nodes=(3, 4), section_id=3, geom_transf="Linear"),
+            ElasticBeamColumn(id=6, name="B2", nodes=(5, 6), section_id=3, geom_transf="Linear"),
+        ],
+        analyses=[
+            ModalCase(id=1, name="Modal-2", n_modes=NUM_MODES),
+        ],
+    )
+
+
+def main() -> None:
+    project = build_eigen_two_storey_shear_frame()
+    project.validate_references()
+    print(f"Built '{project.meta.name}'")
+    print(f"  Reference Tcl: {REFERENCE_TCL.name}")
+    print(f"  Nodes: {len(project.nodes)}, elements: {len(project.elements)}")
+    print(f"  MP constraints: {len(project.mp_constraints)}, modes: {NUM_MODES}")
+    out_path = Path(__file__).with_suffix(".osmodel")
+    save_project(project, out_path)
+    print(f"Saved -> {out_path}")
+    restored = load_project(out_path)
+    restored.validate_references()
+    assert restored.model_dump(by_alias=True) == project.model_dump(by_alias=True)
+    print("Round-trip OK.")
+
+
+if __name__ == "__main__":
+    main()