Import hook: verify library registered, re-entry guard

- _import_hook verifies library_loaded_p AFTER load_library_file
  to catch cases where the file loads but define-library doesn't register
- Re-entry guard (_loading_libs) prevents infinite retry loops
- cek_run import patch deferred — retry approach infinite-loops because
  cek_step_loop re-enters deeply nested eval contexts. Root cause:
  eval_expr → cek_run → cek_step_loop processes the ENTIRE remaining
  kont chain after import resolution, which includes rendering code that
  triggers MORE eval_expr calls. Needs architectural solution (step-level
  suspension handling, not run-level).

Server runs with 4 harmless IO suspension errors. These don't affect
functionality — symbols load via the global env.

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

hosts/ocaml/bin/sx_server.ml

@@ -807,7 +807,9 @@ let () =
     Hashtbl.replace _loading_libs key true;
     let result = match resolve_library_path lib_spec with
       | Some path ->
-        (try load_library_file path; true
+        (try load_library_file path;
+          (* Verify the library actually registered *)
+          Sx_types.sx_truthy (Sx_ref.library_loaded_p lib_spec)
          with e ->
           Printf.eprintf "[import-hook] FAIL %s from %s: %s\n%!"
             key path (Printexc.to_string e);