RLMESH_ABI_VERSION
Binary ABI generation — bumped ONLY on a binary-incompatible change (a repr(C) layout/enum-discriminant change, an extern "C" signature retype, or a symbol removal). Decoupled from the package semver below, which can't express an ABI break. Appending a struct_size-guarded vtable/config field is NOT a break.
#define RLMESH_ABI_VERSION 5