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_document_editor_scroll.py

@@ -9,12 +9,12 @@ document.js is browser-coupled and not importable in pytest.
 """
 
 from pathlib import Path
+from tests.helpers.stylesheets import app_css
 from tests.helpers.document_source import document_source
 
-
 ROOT = Path(__file__).resolve().parents[1]
 DOC_JS = document_source()
-STYLE_CSS = (ROOT / "static/style.css").read_text(encoding="utf-8")
+STYLE_CSS = app_css()
 
 
 def test_document_textarea_scrollbar_is_visible():