RACE: Semantic-linker approach to stable cross-crate ABI without runtime overhead

The BTF parallel is apt - CO-RE (Compile Once, Run Everywhere) in BPF solves a similar relocation problem with type metadata, and it works well in practice. Explicit opt-in marking is a compelling constraint - it would directly address the const expression concern raised above, since only explicitly marked types would participate in layout deferral. This could be the right shape for a minimal first version