Function codegen_native_tier_export 

pub fn codegen_native_tier_export(
    name: Symbol,
    params: &[(Symbol, &TypeExpr<'_>)],
    return_type: Option<&TypeExpr<'_>>,
    interner: &Interner,
) -> Option<String>

Emit the AOT-native export shim (HOTSWAP §Axis-3): a thin #[no_mangle] extern "C" calling-convention wrapper over the inner Rust fn that crosses our ACTUAL values — scalars BY VALUE — with NO CString/handle marshaling (unlike [codegen_c_export_with_marshaling]). The interpreter marshals VM Values to these scalar args, calls the loaded symbol, and re-boxes the scalar result.

Returns None unless every param AND the return are in the sound scalar subset ([is_native_scalar]): the caller then falls through to VM+JIT, so a function the shim cannot represent simply isn’t AOT-native — no gap at the seam.