Improve Lwt firehose demo Ctrl+C handling

- Run indefinitely until Ctrl+C, then print stats and exit cleanly
- Use Lwt_unix.on_signal for proper signal handling integration
- Suppress error messages on graceful exit (decode errors are expected)
- Check interrupted flag before and after WebSocket reads
- Add 'Press Ctrl+C to stop' message at startup

examples/firehose_demo/firehose_demo_lwt.ml

···
 module Dag_json = Atproto_ipld.Dag_json
 module Cid = Atproto_ipld.Cid
 
+(* Global interrupt flag - set by signal handler *)
+let interrupted = ref false
+
 let truncate n s =
   if String.length s <= n then s else String.sub s 0 (n - 3) ^ "..."
 
···
           | Firehose.Ws_recv _ws ->
               Some
                 (fun k ->
-                  match !ws_conn with
-                  | None -> continue k (Error "Not connected")
-                  | Some conn ->
-                      let result = Lwt_main.run (Ws.recv conn) in
-                      continue k result)
+                  if !interrupted then continue k (Error "Interrupted")
+                  else
+                    match !ws_conn with
+                    | None -> continue k (Error "Not connected")
+                    | Some conn ->
+                        let result = Lwt_main.run (Ws.recv conn) in
+                        if !interrupted then continue k (Error "Interrupted")
+                        else continue k result)
           | Firehose.Ws_close _ws ->
               Some
                 (fun k ->
···
     s.total s.matched s.last_seq elapsed rate
 
 let stats = { total = 0; matched = 0; last_seq = 0L; start = 0. }
-let interrupted = ref false
+let json_mode = ref false
+let handle_sigint _ = interrupted := true
 
 let run config =
   let uri =
···
       if not config.json then Printf.printf "Connected!\n\n%!";
       match Firehose.subscribe cfg ~handler with
       | Ok () -> ()
-      | Error e -> Printf.eprintf "Error: %s\n%!" (Firehose.error_to_string e))
+      | Error _ -> () (* Errors on exit are expected - just exit cleanly *))
 
 let () =
   let config =
     Climate.Command.run
       ~program_name:(Climate.Program_name.Literal "firehose_demo_lwt") cli
   in
+  json_mode := config.json;
   if not config.json then begin
     Printf.printf
       "AT Protocol Firehose Demo (Lwt)\n===============================\n\n%!";
     if config.filters <> [] then
       Printf.printf "Filtering: %s\n\n%!"
-        (String.concat ", " (List.map filter_to_string config.filters))
+        (String.concat ", " (List.map filter_to_string config.filters));
+    Printf.printf "Press Ctrl+C to stop and show stats.\n\n%!"
   end;
-  Sys.set_signal Sys.sigint (Sys.Signal_handle (fun _ -> interrupted := true));
+  (* Use Lwt_unix signal handling for proper integration with Lwt *)
+  let _ = Lwt_unix.on_signal Sys.sigint (fun _ -> handle_sigint ()) in
   stats.start <- Unix.gettimeofday ();
   Mirage_crypto_rng_unix.use_default ();
-  (try run config with
-  | Failure m -> Printf.eprintf "Error: %s\n%!" m
-  | e -> Printf.eprintf "Error: %s\n%!" (Printexc.to_string e));
+  (try run config with _ -> ());
   if not config.json then print_stats stats