Merge lab into test/document-module-set-helper

Resolve the overlap with #19 by preserving the whole-cascade stylesheet
helpers alongside #31's document-source/module-set helpers.

Maintainer validation:
- document/module contract and composition guards: 15 passed
- all 54 touched Python test modules: 366 passed
- py_compile: clean
- git diff --check: clean

tests/test_selection_overlay_clear_static.py

@@ -1,4 +1,5 @@
 from pathlib import Path
+from tests.helpers.stylesheets import app_css
 from tests.helpers.document_source import document_source
 
 
@@ -7,7 +8,7 @@ ROOT = Path(__file__).resolve().parents[1]
 
 def test_selection_overlays_have_individual_clear_controls():
     js = document_source()
-    css = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    css = app_css()
 
     assert "function clearSelectionAt(index)" in js
     assert "className = 'doc-selection-overlay-clear'" in js