Configurable gap between stack limit and store

If the stack can grow down to `lim`, and the first store value is at `lim`-2048, then an interrupt or signal handler can clobber data within a 2048-byte region immediately below the stack pointer at any point without interfering with state. I think this is the only thing that needs to be implemented and proved for #757 to be usable in general environments.

Start by reading issue #757 and tracing the stack-limit and first-store behavior it introduces. Determine where the 2048-byte gap is represented and where its safety is proved; done means the gap is configurable and the proof establishes that interrupt or signal-handler writes in that region do not interfere with program state.

Written by the indexing model from the issue text.