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_runtime.ml

@@ -102,7 +102,8 @@ let get_val container key =
   | Dict d, Keyword k -> dict_get d k
   | (List l | ListRef { contents = l }), Number n ->
     (try List.nth l (int_of_float n) with _ -> Nil)
-  | _ -> raise (Eval_error ("get: unsupported " ^ type_of container ^ " / " ^ type_of key))
+  | Nil, _ -> Nil  (* nil.anything → nil *)
+  | _, _ -> Nil    (* type mismatch → nil (matches JS/Python behavior) *)
 
 (** Register get as a primitive override — transpiled code calls (get d k). *)
 let () =