Fallible MemoryPool claim and reserve methods?

### Is your feature request related to a problem or challenge?

Under strict memory limits, it seems there would currently be a time-of-check vs time-of-use issue between checking `available` memory vs `reserve`ing it. Fallible `reserve` and `claim` methods could avoid that.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by tracing the MemoryPool APIs and the existing available, reserve, and claim methods. Clarify the intended fallible behavior and how strict memory limits should be handled; done means the TOCTOU concern is resolved with an agreed API and demonstrated behavior.

Written by the indexing model from the issue text.