The root compose recorded seams last-writer-wins, so the status
container's new seam (mounted loader) overwrote the transcript's seam
whenever both reported in the same frame. With a volatile live block
(no commit-safe end) the boundary then moved below the transcript's
still-mutable rows and committed them as stale history — the same
duplication class this PR fixes for the loader, relocated upward.
Commits are prefix-only: the first child reporting a live region (plus
its own commit-safe extension) already bounds everything below it, so
the topmost seam now wins. Adds a two-reporter regression test and
documents the multi-reporter contract on NativeScrollbackLiveRegion.
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