v0.12.143: @newsroom.writer: trail ends, the next piece where the article ends

An article that is part of a trail opens with a row of cards, this one marked "you are here". At the end of the text the only way on was "Continued by", under the reading meter. Now the same cards, smaller, close the article text, just before the meter, with the next piece marked "next". 27 articles have one, among them the Pellbridge and Thistlewood pieces.

admin/build/trail_end.py copies the cards from the page's own trail, so trails published by sgit.ai work too. series.py runs it, so every release that stamps series stamps trail ends. The block is <nav class="trail-end">: summaries.py stops reading an article at it, so no section's text or hash changes; validate.js and sync_newsroom.py strip it, as they strip the series blocks; the search index skips it. The styles are in assets/site-nav.css; on a phone the cards drop their images and stack.