franksong2702: preserve the reader's semantic viewport anchor during mid-stream transcript rebuilds
before raw scrollTop fallback; remount virtualized anchors before restore w/ _programmaticScroll
scoped to the remount frame; treat captured userUnpinned/pinned snapshot as authoritative so
distance-inference can't re-pin a reader who scrolled away during a live reply. Browser-only scroll
state. Co-authored-by: franksong2702 <franksong2702@users.noreply.github.com>
