`AccumulatorMinMaxN::_processValue()`: ($minN/$maxN):

• `val.getApproximateSize()` was computed unconditionally on every call, but that size is only needed when the value is actually stored. In steady state the heap is full of n elements and most incoming values are rejected by the `_valueComp.compare(...)` check. Since `getApproximateSize()` recursively walks the `Value`, this was wasted work for every rejected value.
• On eviction, replaced `_heap.back() = {val, sz}

  ` which builds a temporary pair then move-assigns) with direct member assignment.

`AccumulatorFirstLastN::_processValue()`: ($firstN/$lastN):

• The `valToProcess` copy of `val` was materialized before the $firstN- already-full early return, so it was copied and immediately discarded. The copy is now made after the early-return check.