Make replay budget degradation monotonic
When the newest turn alone exceeded the 100k replay budget it fell back
to flat without consuming the budget, so an older, smaller turn could
still expand: rich tool history for a stale turn while the turn being
continued from was plain prose, inverting the newest-first invariant.
Once any turn falls back to flat, every older turn now does too, with a
regression test where the newest turn alone exceeds the budget.
