[ilyapt]

scenario	before, ns/op	after, ns/op	allocs/op before → after
rest on an existing level + cancel	20	15	0 → 0
rest on a new level + cancel	70	64	0 → 0
fill one maker + replenish	36	23	1 → 0
sweep 10 makers + replenish	356	237	5 → 0
sweep 100 makers + replenish	3 633	3 414	8 → 0
FOK rejected after walking 100 levels	244	234	0 → 0
PostOnly rejected	7.5	5.7	0 → 0
mixed: 70% rest, 25% fill one, 5% sweep 10	45	33	0.3 → 0

Trades no longer allocate: they go to a TradeSink one by one instead of being appended to a slice in the report. Reserving the order slot before matching costs nothing measurable.