Note power-of-2 requirement to match new design text

ml-proto/src/spec/eval.ml

@@ -263,7 +263,7 @@ let init_memory ast =
 
 let init m imports host =
   assert (List.length imports = List.length m.it.Ast.imports);
-  assert (host.page_size > 0);
+  assert (host.page_size > 0 and Int.is_power_of_two host.page_size);
   let {Ast.exports; globals; tables; funcs; memory; _} = m.it in
   let mem = init_memory memory in
   let func x = List.nth funcs x.it in