Spin echo visibility grows only when the recoverable bucket grows. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Recoverable and irreversible buckets trade off inside one budget
Initial transverse budget is 15 and coherent bucket is fixed at 5.
echo visible=coherent+recoverable
Three split rows keep the initial budget closed
The irreversible bucket stays outside the echo-visible result.