Merge remote-tracking branch 'fork/v1/css-split-3' into lane/decomposition-and-test-isolation

tests/README.md

@@ -172,6 +172,23 @@ The helpers below live under `tests/helpers/`. They exist to remove repeated
 boilerplate that already appeared across multiple tests. Reach for one only when
 your test matches its intended use; do not stretch a helper to cover a new case.
 
+### `tests.helpers.stylesheets.app_css`
+
+Use when a test asserts on a CSS rule.
+
+- Returns every app stylesheet concatenated in the order `static/index.html`
+  loads them, which is the order the cascade actually has.
+- Panel styles no longer all live in `static/style.css`; reading that file
+  alone ties the test to whichever file a rule sits in today, so it goes red
+  when a rule moves without the rendered page changing.
+- `stylesheet_paths()` and `stylesheet_urls()` are there when a test needs the
+  files or the request URLs rather than their contents.
+  `stylesheet_link_tags()` returns the `<link>` markup for a synthetic page
+  driven through Playwright, so it gets the whole cascade instead of only
+  `style.css`.
+- All of them fail loudly if `index.html` links a stylesheet that is missing.
+- Not for vendored CSS under `static/lib/`, which they deliberately skip.
+
 ### `tests.helpers.cli_loader.load_script`
 
 Use when a test needs to import a script under `scripts/` without repeating