validNativeBinding ABI validation

Trap: accepting ELF/Mach-O magic or the first MZ bytes looks like native-format validation, but a truncated or wrong-architecture addon passes and fails only at dlopen; accepting any sole napi-v<N> directory likewise permits an unsupported ABI. Fix: validate minimum header size, ELF class/machine, Mach-O CPU, PE signature/machine, and runtime-supported N-API version. Test real generated binaries for every declared target; headers validate compatibility, not authenticity.