Constant SPILL_MIN_BYTES 

const SPILL_MIN_BYTES: usize = _; // 65_536usize

Bodies smaller than this stay resident: the pool’s smallest size class is 64 KiB, so spilling below it trades no meaningful memory for slot waste.

Sub-floor bodies are invisible to the pool’s budget, which is safe only while they are rare. settle coalesces toward COMMIT_BYTES before committing, so in the harness only a final done tail commits below the floor. A caller that commits many small chunks directly accumulates unbudgeted heap, and no accounting here would catch it.