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/unit/test_meta_gates.py

@@ -0,0 +1,29 @@
+"""Meta-gates tying the test markers to the CI commands that consume them."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+try:
+    import tomllib
+except ModuleNotFoundError:  # Python < 3.11
+    import tomli as tomllib
+
+ROOT = Path(__file__).resolve().parents[2]
+PYPROJECT = ROOT / "pyproject.toml"
+CI = ROOT / ".github" / "workflows" / "ci.yml"
+
+HEADLESS_COMMAND = 'pytest tests/unit tests/services -m "not slow"'
+
+
+def test_slow_marker_is_declared() -> None:
+    """``[tool.pytest.ini_options]`` registers the ``slow`` marker."""
+    with PYPROJECT.open("rb") as handle:
+        config = tomllib.load(handle)
+    markers = config["tool"]["pytest"]["ini_options"]["markers"]
+    assert any(marker.startswith("slow:") for marker in markers), markers
+
+
+def test_ci_runs_the_headless_command() -> None:
+    """The headless CI command matches the marker the suite expects."""
+    assert HEADLESS_COMMAND in CI.read_text(encoding="utf-8")