Supersedes the v0.51.199 proximity-re-pin (#3330) and the #3250 upward-intent
timeout with a sticky-unpin model (ChatGPT/Claude/Codex behavior): scroll up =
stay put until you return to the bottom or click the scroll-to-bottom control.
Reconciled against the shipped #3330 code: removed the now-dead
_recentMessageUpwardIntent reference from the #3319 rAF retry, kept the
load-time -Infinity intent-init fix, kept the pinned-only >500 catch-up.
Co-authored-by: pamnard <pamnard@users.noreply.github.com>