TODO: give back memory on real heap

This should probably work with just `sbrk(negative_num)`
...I suppose

Also come up with a strategy when to do this.
Maybe when freed > alloced? This will require another callback in the allocators. For the static heap we'll need a dummy callback.

Start by locating the real-heap path that uses sbrk and the allocator callbacks mentioned in the issue. Read how the static heap and allocators currently handle freeing, then determine a strategy for when negative sbrk should be called and how the static heap's dummy callback fits. Done means real-heap memory is returned safely without breaking static-heap behavior.

Written by the indexing model from the issue text.