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_edit_reference_js.py

@@ -1,13 +1,14 @@
 """Regression guards for document-selection references in chat bubbles."""
 
 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]
 RENDERER = (ROOT / "static/js/chatRenderer.js").read_text(encoding="utf-8")
 DOCUMENT = document_source()
-STYLE = (ROOT / "static/style.css").read_text(encoding="utf-8")
+STYLE = app_css()
 INDEX = (ROOT / "static/index.html").read_text(encoding="utf-8")
 APP = (ROOT / "static/app.js").read_text(encoding="utf-8")