Constant NATIVE_P256_BACKEND 

pub const NATIVE_P256_BACKEND: bool = true;

true when NativeCrypto routes secp256r1_verify through the native aws-lc-rs backend; false when it falls back to the portable p256 trait default (e.g. zkVM guest builds). Differential tests assert this so they fail loudly instead of silently comparing the pure-Rust backend to itself.