Does the stabilization of Kotlin Multiplatform include stabilization or guarantees of the C++ runtime for K/N? Can I, for example, try to call parts of the runtime manually, or make assumptions about the binary structure of a Kotlin class?

Sorry, no stable API is planned for the runtime. And the binary representation of classes (of both types and objects) may (and likely will) change with time as we look for more performant designs. What is your use case for this, by the way?

I was considering a project that would require me to change elements of the runtime. I started a prototype around 1.6.0's release, but there were major changes that broke what I'd had, so I wanted to wait until the runtime is more stabilized.