The disk-fullness gate only rejected destinations already at/above the mark, so a
server just under it could take a large volume and overshoot. Project the selected
volume's bytes onto the candidate: if the move would cross the mark, drop that
destination for the rest of the cycle and re-pick instead of overshooting. Also
note the per-location capacity-summing assumption on the Rust heartbeat side, to
match the Go store.go comment.