include/ravel/ravel.h is the C ABI surface. Until 1.0.0, it can break between minor versions — every break must be called out in the release notes. From 1.0.0 on:

Breaking (major version bump): removing or renaming an exported function, changing a function's signature, changing the layout/size of any struct passed by value across the boundary (none yet — ravel_simulation is opaque on purpose, keep it that way rather than exposing a struct layout).
Non-breaking (minor version bump): adding a new exported function, adding a new opaque handle type.
Patch version: behavior fixes that don't change the signature surface.

Keep every ABI type opaque (ravel_simulation*, not a struct with public fields) specifically so internal layout can change without an ABI break — this is why ravel_simulation is a forward-declared pointer, not a struct the caller allocates.