Give the menu's headers the cap it now draws

Plan step 36. The rewrite drew the popover's 18rem cap as 288px, and its
stylesheet opened with "two lists" before naming three; the headers now
say what the rules do.

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

resources/css/components/menu.css

@@ -2,7 +2,7 @@
  * The menus: `<x-menu>` and what a form's own dropdowns wear too, so a menu and a form read as
  * one family.
  *
- * Two lists wear the dropdown:
+ * Three lists wear it:
  *
  *   - `<x-menu>`'s own popover (`[data-md-menu-popover]`) and, inside it, a submenu
  *     (`[data-md-submenu]`, menu-item.css).
@@ -19,7 +19,7 @@
  * focus.
  *
  * `[data-md-menu]` is `<x-menu>`'s root, holding the trigger and the popover; the popover itself
- * is `[data-md-menu-popover]`, capped at 18rem so a long menu scrolls instead of running off the
+ * is `[data-md-menu-popover]`, capped at 288px so a long menu scrolls instead of running off the
  * top layer's edge (ACT-04), and transitions in by growing from its trigger's corner while it
  * fades, on the spatial and effects springs respectively (ACT-26). `data-md-vibrant` swaps its
  * container for tertiary-container; a nested submenu inherits the colour through