VM upvalues + HO primitives + 40 tests (36 pass, 4 fail)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_runtime.ml

@@ -46,12 +46,15 @@ let sx_call f args =
   | Lambda l ->
     let local = Sx_types.env_extend l.l_closure in
     List.iter2 (fun p a -> ignore (Sx_types.env_bind local p a)) l.l_params args;
-    (* Return the body + env for the trampoline to evaluate *)
     Thunk (l.l_body, local)
   | Continuation (k, _) ->
     k (match args with x :: _ -> x | [] -> Nil)
   | _ -> raise (Eval_error ("Not callable: " ^ inspect f))
 
+(* Initialize forward ref so primitives can call SX functions *)
+let () = Sx_primitives._sx_call_fn := sx_call
+(* Trampoline ref is set by sx_ref.ml after it's loaded *)
+
 (** Apply a function to a list of args. *)
 let sx_apply f args_list =
   sx_call f (sx_to_list args_list)