Say why the FAB menu's exit rule wins inside the layer

Plan step 36. fab-menu.css claimed the item sets no translate or opacity
of its own once it has entered; fab-menu-item.css sets both at rest, and
the closing rule wins on specificity, which the header now says.
fab-menu-item.css named the rule by the root's hook instead of the
popover's, and a render test named for the list's scroll asserted the
exit; it is named for what it checks.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Feature/Components/FabMenuTest.php

@@ -65,7 +65,7 @@ it('opens at the corner asked for, secondary and tertiary in their own colours',
         ]);
 });
 
-it('scrolls the list on a short window, behind the close button, which stays fixed', function () {
+it('sinks the items back while the list closes, from the popover\'s own open state', function () {
     expect(ComponentStylesheet::read('fab-menu')->declarations('[data-md-fab-menu-popover]:not(:popover-open) > *'))->toBe([
         'translate' => '0 var(--md-sys-measurement-space100)',
         'opacity' => '0',