Function io_backend_type 

pub fn io_backend_type() -> u32

Returns the I/O backend type code used for DiskAnn disk reads.

Compare against the ZVEC_IO_BACKEND_TYPE_* constants in crate::sys: Linux selects io_uring, then libaio, then synchronous pread; macOS ARM64 uses pread; Windows uses overlapped I/O. Use io_backend_type_name for the matching name.