Lead section detection broken under both mw-heading and Parsoid

[HakanIST]

The > h2 selector used to find the first heading (line 271) no longer works:

elements = $( '.mw-parser-output > h2' ).first().prevAll();

1. Legacy parser: headings are now wrapped in div.mw-heading, so there are no h2 elements as direct children of .mw-parser-output.
2. Parsoid (T430194): content is wrapped in <section> elements (T333031), so neither headings nor the short description are direct children.

In both cases elements is empty, so section stays undefined and the full page wikitext is always downloaded instead of just section 0. Not a functional break, but an unnecessary performance hit on every page load.