JIT: skip recompilation for known-failed closure names

New closure instances of functions like parse-loop get l_compiled=None
even though a previous instance already failed. Now check _jit_warned
by name and immediately sentinel-mark new instances of known failures.

The first failure is always logged. Subsequent instances are silently
sentinel-marked — no recompilation, no log spam, no suppressed errors.

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

hosts/ocaml/bin/sx_server.ml

@@ -728,6 +728,8 @@ let register_jit_hook env =
        | Some _ -> None  (* compile failed or disabled — CEK handles *)
        | None ->
          if !_jit_compiling then None
+         else if Hashtbl.mem _jit_warned fn_name then
+           (l.l_compiled <- Some Sx_vm.jit_failed_sentinel; None)
          else begin
            _jit_compiling := true;
            let t0 = Unix.gettimeofday () in