Fix get nil-safety in sx_runtime.ml + reduce VM failure log noise

The second get implementation in sx_runtime.ml (used by transpiled code)
was still raising on type mismatches. Now returns nil like sx_primitives.

Remove per-call [vm-call-closure] FAIL logging — the jit-hook already
logs failures at the right level. Reduces 70K log lines to ~5.

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

hosts/ocaml/lib/sx_vm.ml

@@ -381,13 +381,7 @@ and call_closure cl args globals =
   List.iter (fun a -> push vm a) args;
   for _ = List.length args to cl.vm_code.vc_locals - 1 do push vm Nil done;
   vm.frames <- [frame];
-  (try run vm
-   with e ->
-     Printf.eprintf "[vm-call-closure] FAIL in %s: %s (bc_len=%d args=%d sp=%d)\n%!"
-       (match cl.vm_name with Some n -> n | None -> "?")
-       (Printexc.to_string e)
-       (Array.length cl.vm_code.vc_bytecode) (List.length args) vm.sp;
-     raise e);
+  (try run vm with e -> raise e);
   pop vm
 
 (** Execute a compiled module (top-level bytecode). *)