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_compare_mobile_tabs_static.py

@@ -1,4 +1,5 @@
 from pathlib import Path
+from tests.helpers.stylesheets import app_css
 
 
 ROOT = Path(__file__).resolve().parent.parent
@@ -32,7 +33,7 @@ def test_mobile_compare_reconciles_tabs_after_pane_lifecycle_changes():
 
 
 def test_mobile_compare_css_shows_only_the_active_card():
-    css = _read("static/style.css")
+    css = app_css()
     mobile = css[css.index("/* Compare uses one full-width response card on phones."):]
 
     assert ".compare-mobile-tabs" in mobile
@@ -44,7 +45,7 @@ def test_mobile_compare_css_shows_only_the_active_card():
 
 def test_probe_feedback_and_actions_use_the_shared_card_layout():
     selector = _read("static/js/compare/selector.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "probeFeedback.className = 'compare-probe-feedback'" in selector
     assert "probeFeedback.appendChild(detail)" in selector
@@ -66,7 +67,7 @@ def test_probe_feedback_and_actions_use_the_shared_card_layout():
 
 def test_probe_swap_reopens_and_highlights_the_failed_model_slot():
     selector = _read("static/js/compare/selector.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "function _expandModelSlot(slotIdx)" in selector
     assert "row.dataset.slotIndex = String(idx);" in selector