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

resources/css/components/fab-menu-item.css

@@ -7,7 +7,7 @@
  * container.
  *
  * The entry is `@starting-style` on the item itself; the exit needs the state to be readable
- * while the popover is on its way out, which `[data-md-fab-menu]:not(:popover-open) > *`
+ * while the popover is on its way out, which `[data-md-fab-menu-popover]:not(:popover-open) > *`
  * (fab-menu.css) drives from the popover's own open state — the popover keeps `display: flex` for
  * the length of its own discrete transition, so the item has somewhere to animate to (ACT-33).
  */