.btn-split--hidden had no matching CSS rule anywhere, so the opt-in Split
header button shipped visible to every user on every viewport regardless of
the setting. Add the `display: none !important` rule alongside its sibling
marker classes (.btn-multimonitor--hidden etc.), plus a static regression
guard (test/split-pane-hidden-button-css.test.ts) that fails if any future
"*--hidden" marker class in index.html is missing a matching CSS rule.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
