Split renderFeed() into full rebuild + prependPostCard() on both feed.html
and post.html. New live events now prepend a single card at the top of the
feed without rebuilding existing cards, eliminating O(n) DOM rebuilds on
every incoming event. DOM is trimmed to displayCount on overflow with
renderedPostIds kept in sync. Full rebuild path preserved for See More,
mute removal, bootstrap completion, and event mode.
Also added implementation checklist to plans/long-running-memory-bloat-fix.md.
