loadThrough now assigns its low-water target only when it owns the loop
(retargeting stays inside the running-jump branch): a call refused while
a plain load-earlier pull holds the pager no longer leaves jumpTargetSeq
behind to drag a later jump all the way to the head. The loop also
carries the doOpen stale-pass guard so a mid-flight resync stops it
instead of paging the new stream generation toward the old target.
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