refactor(tests): read the whole cascade instead of style.css alone

static/style.css no longer holds every rule. 79 rules for the document,
gallery and editor panels now live in static/css/, yet 27 browser tests still
built their synthetic page with a single <link> to style.css, and 51 more read
that one file as though it were the whole cascade. Those tests kept passing
while covering less: a rule that moved became invisible to the assertion that
was meant to pin it.

Python readers now call tests.helpers.stylesheets.app_css(), and synthetic
pages are built from stylesheet_link_tags() so they load exactly what
index.html loads, in the same order. The helper already existed; this moves
the remaining callers onto it.

test_portal_dropdown_z_js parametrised over a file list including style.css to
assert an absence. Checking a negative against one file of a split stylesheet
is how a moved rule escapes, so the CSS case now checks the concatenation.

Two guards keep it from coming back: one fails on any test reading
static/style.css directly, the other on any synthetic page linking it alone.
Both name the helper to use.

No production code changes. Suite is unchanged at 6 pre-existing failures.

tests/test_document_rich_structure_tools.py

@@ -3,11 +3,13 @@
 import json
 import subprocess
 from pathlib import Path
+from tests.helpers.stylesheets import app_css
+from tests.helpers.stylesheets import stylesheet_link_tags
 
 
 ROOT = Path(__file__).resolve().parents[1]
 DOC_JS = (ROOT / "static/js/document.js").read_text(encoding="utf-8")
-STYLE = (ROOT / "static/style.css").read_text(encoding="utf-8")
+STYLE = app_css()
 
 
 def test_heading_levels_and_page_break_are_exposed_everywhere():
@@ -44,7 +46,7 @@ def test_mobile_headings_page_break_history_and_persistence():
       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&structure-tools-test=1');
         window.__structureToolsDocModule = mod;
@@ -152,6 +154,7 @@ def test_mobile_headings_page_break_history_and_persistence():
       }));
       await browser.close();
     """
+    script = script.replace("__ODY_STYLESHEETS__", stylesheet_link_tags())
     result = subprocess.run(
         ["node", "--input-type=module", "-e", script],
         cwd=ROOT,