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_docs.py

@@ -0,0 +1,51 @@
+"""Documentation gates — assert the user-facing docs keep their key content.
+
+Pure pathlib; no Qt or solver imports so this stays part of the fast unit
+suite.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+DOCS_DIR = REPO_ROOT / "docs"
+
+
+def _text(path: Path) -> str:
+    """Return a file's UTF-8 text with normalized line endings."""
+    return path.read_text(encoding="utf-8").replace("\r\n", "\n")
+
+
+def test_quick_guide_exists_and_covers_core_topics() -> None:
+    quick_guide = DOCS_DIR / "QUICK_GUIDE.md"
+    assert quick_guide.is_file(), "docs/QUICK_GUIDE.md is missing"
+
+    lowered = _text(quick_guide).lower()
+    for topic in ("cantilever", "report", "modal", "units", "undo"):
+        assert topic in lowered, f"QUICK_GUIDE.md is missing {topic!r}"
+
+
+def test_contributing_describes_this_repo() -> None:
+    text = _text(REPO_ROOT / "CONTRIBUTING.md")
+
+    for required in (
+        "views",
+        "viewmodels",
+        "services",
+        "core",
+        "commands",
+        "Conventional Commits",
+        "feat:",
+        "main",
+        "pre-commit install",
+        'pip install -e ".[gui,dev]"',
+        "python -m otko",
+        'pytest -m "not slow"',
+    ):
+        assert required in text, f"CONTRIBUTING.md is missing {required!r}"
+
+
+def test_readme_links_quick_guide() -> None:
+    readme = _text(REPO_ROOT / "README.md")
+    assert "docs/QUICK_GUIDE.md" in readme