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/conftest.py

@@ -0,0 +1,30 @@
+"""Shared pytest configuration.
+
+The GUI suite is headless by default: unless the caller already chose a
+platform, ``QT_QPA_PLATFORM=offscreen`` is set here, before pytest-qt
+builds its session ``QApplication``. An explicit caller value is respected.
+
+OpenseesPy is a process-global singleton, so a lazy autouse guard wipes the
+domain after every test. The import is inside the teardown so that
+``tests/unit`` never imports the solver.
+"""
+
+from __future__ import annotations
+
+import os
+from collections.abc import Iterator
+
+import pytest
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+
+@pytest.fixture(autouse=True)
+def _wipe_opensees_domain() -> Iterator[None]:
+    """Wipe the OpenseesPy domain after a test if the solver was imported."""
+    yield
+    try:
+        from openseespy.opensees import wipe
+    except Exception:  # pragma: no cover - solver not installed / import side effects
+        return
+    wipe()