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/ex4_portal2d_inelastic_fiber_section.py

@@ -0,0 +1,56 @@
+"""OpenSees Example 4. Portal Frame: fiber-section build."""
+
+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"))
+    sys.path.insert(0, str(Path(__file__).resolve().parent))
+
+from otko.services import load_project, save_project  # noqa: E402
+
+if __package__:
+    from ._ex4_portal2d_common import (  # noqa: E402
+        ANALYSIS_DT,
+        ANALYSIS_STEPS,
+        FIBER_VARIANT,
+        PUSH_STEP,
+        PUSH_TARGET,
+        REFERENCE_PUSH_TCL,
+        REFERENCE_SINE_TCL,
+        build_ex4_portal2d_inelastic_fiber_section,
+    )
+else:
+    from _ex4_portal2d_common import (  # noqa: E402
+        ANALYSIS_DT,
+        ANALYSIS_STEPS,
+        FIBER_VARIANT,
+        PUSH_STEP,
+        PUSH_TARGET,
+        REFERENCE_PUSH_TCL,
+        REFERENCE_SINE_TCL,
+        build_ex4_portal2d_inelastic_fiber_section,
+    )
+
+
+def main() -> None:
+    project = build_ex4_portal2d_inelastic_fiber_section()
+    project.validate_references()
+    print(f"Built '{project.meta.name}'")
+    print(f"  Build Tcl: {FIBER_VARIANT.build_tcl_name}")
+    print(f"  Analysis Tcls: {REFERENCE_PUSH_TCL.name}, {REFERENCE_SINE_TCL.name}")
+    print(f"  Pushover target = {PUSH_TARGET}, step = {PUSH_STEP}")
+    print(f"  Sine transient: dt = {ANALYSIS_DT}s, steps = {ANALYSIS_STEPS}")
+    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()