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_shuffle_visual_static.py

@@ -1,5 +1,6 @@
 from pathlib import Path
 import re
+from tests.helpers.stylesheets import app_css
 
 
 ROOT = Path(__file__).resolve().parent.parent
@@ -11,7 +12,7 @@ def _read(path: str) -> str:
 
 def test_compare_shuffle_shows_center_notice_with_dice_icon():
     panes = _read("static/js/compare/panes.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "ICON_DICE" in panes
     assert "compare-shuffle-notice" in panes
@@ -23,7 +24,7 @@ def test_compare_shuffle_shows_center_notice_with_dice_icon():
 def test_compare_chat_and_agent_panes_expose_per_pane_inference_settings():
     index = _read("static/js/compare/index.js")
     panes = _read("static/js/compare/panes.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "pane-settings-btn" in panes
     assert "paneSettingsButtonHtml" in index
@@ -41,7 +42,7 @@ def test_compare_chat_and_agent_panes_expose_per_pane_inference_settings():
 def test_compare_probe_control_has_requested_vertical_alignment():
     index = _read("static/js/compare/index.js")
     probe = _read("static/js/compare/probe.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert 'class="compare-check-icon"' in index
     assert '<span class="compare-check-label">Probe</span>' in index
@@ -75,7 +76,7 @@ def test_compare_cache_key_bumped_for_shuffle_notice():
 
 def test_compare_score_button_label_is_nudged_up():
     vote = _read("static/js/compare/vote.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert '<span class="compare-score-label">Score</span>' in vote
     assert ".compare-score-label" in css