fix: let the mouse wheel scroll the reading viewport

overscroll-behavior: none on the script-scrolled, overflow-hidden reading
viewport swallowed wheel events, so the page only moved by keyboard,
touch or the navigation.

src/styles/world.css

@@ -597,7 +597,8 @@
   /* A real scroll container (scrolled from script, never by the user), so
      sticky children and view timelines resolve against the reading window. */
   overflow: hidden;
-  overscroll-behavior: none;
+  /* Wheel and touch scrolling must chain through to the document, which
+     drives this container: no overscroll-behavior here. */
   isolation: isolate;
 }