Macro __impl_alt_elt 

#[doc(hidden)]
macro_rules! __impl_alt_elt {
    ($ty:ty, $trait:path, $elem:ty, $na:expr) => { ... };
}

Shared elt implementation for ALTREP families with direct element access.

Generates const HAS_ELT and fn elt(...) inside an impl block. Used by integer, real, raw, and complex families.