Function compile_nested 

pub fn compile_nested(
    ctx: &Context,
    decl: &NestedDecl,
) -> KernelResult<Compiled>

Compile a nested inductive into a mutual block plus conversion isos. Untrusted: every artifact is checked by the caller. Handles arbitrary nesting DEPTH (TList (TList RTree)) by recursive specialization — each container level becomes its own mutual sibling.