TUI.render merged live-region seams with a topmost-seam-wins rule that
adopted only that seam's pin policy for the whole frame. While the primary
turn streams, the transcript reports the topmost, unpinned seam, so the
frame-wide pin flag becomes false and a pinned AnchoredLiveContainer below
it (the /btw panel, the working HUD) loses its pinning: once its content
grows past the viewport, the emit path commits the scrolled-off rows as
frozen snapshots on every growth frame, piling duplicates into native
scrollback.
Track a pinnedBoundary (start row of the topmost pinned region)
independently of which seam wins the topmost merge, and cap every commit
ceiling at it. Equivalent to the prior behavior for a fully-pinned frame
and for a frame with no pinned region; only the mixed case changes.
Fixes#8793
roboomp
7cff20cfd0