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_rich_table_merge_split.py

@@ -3,6 +3,7 @@
 import json
 import subprocess
 from pathlib import Path
+from tests.helpers.stylesheets import stylesheet_link_tags
 from tests.helpers.document_source import document_source, function_body
 
 
@@ -33,7 +34,7 @@ def test_mobile_merge_split_round_trip_preserves_headers_formatting_and_history(
       const browser = await chromium.launch({ headless: true });
       const page = await browser.newPage({ viewport: { width: 390, height: 844 } });
       await page.goto(`${process.env.ODYSSEUS_TEST_STATIC_ORIGIN}/static/js/documentStats.js`);
-      await page.setContent('<link rel="stylesheet" href="/static/style.css?v=20260831richtexttools91"><div id="toast"></div><div id="chat-container"></div><div id="sidebar"></div>');
+      await page.setContent('__ODY_STYLESHEETS__<div id="toast"></div><div id="chat-container"></div><div id="sidebar"></div>');
       await page.evaluate(async () => {
         const mod = await import('/static/js/document.js?v=20260831richtexttools91&table-merge-split=1');
         mod.init('/api');
@@ -137,6 +138,7 @@ def test_mobile_merge_split_round_trip_preserves_headers_formatting_and_history(
       }));
       await browser.close();
     """
+    script = script.replace("__ODY_STYLESHEETS__", stylesheet_link_tags())
     result = subprocess.run(
         ["node", "--input-type=module", "-e", script],
         cwd=ROOT,