Follow-up to signed integers

make the internal representation twos-complement, and also add Rust tests? Ultimately if this type exists, it'll be used at the ABI level, so it should Just Work with Rust.

Start by locating the signed-integer type and its internal representation, then trace how that type is exposed at the ABI level. Review the existing test setup before adding Rust coverage. Done means the representation uses two's complement and the type works through the ABI with Rust tests demonstrating the behavior.

Written by the indexing model from the issue text.