fix(firehose_demo_lwt): enable Ctrl+C interrupt handling

The Lwt_main.run call blocks and doesn't respond to signals by itself.
This fix uses Lwt.pick to race the WebSocket recv against an interrupt
waiter that gets woken when SIGINT is received.

- Add trigger_interrupt() that wakes up the interrupt waiter
- recv() now races recv_impl() against the interrupt signal
- Signal handler calls trigger_interrupt() instead of just setting flag

Now pressing Ctrl+C properly stops the demo and shows statistics.

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 *)
+(* Global interrupt handling via Lwt *)
+let interrupt_wakener : (unit, exn) result Lwt.u option ref = ref None
 let interrupted = ref false
+
+let trigger_interrupt () =
+  interrupted := true;
+  match !interrupt_wakener with
+  | Some w ->
+      Lwt.wakeup_later w (Error Exit);
+      interrupt_wakener := None
+  | None -> ()
 
 let truncate n s =
   if String.length s <= n then s else String.sub s 0 (n - 3) ^ "..."
···
   let frag_buf = Buffer.create 65536
   let frag_opcode = ref Websocket.Frame.Opcode.Continuation
 
-  let recv conn : (string, string) result Lwt.t =
+  let recv_impl conn : (string, string) result Lwt.t =
     let open Websocket in
     let rec read_binary () =
       Lwt.catch
···
     in
     read_binary ()
 
+  (** Interruptible recv - races against interrupt signal *)
+  let recv conn : (string, string) result Lwt.t =
+    (* Create a waiter that can be woken by Ctrl+C *)
+    let interrupt_waiter, wakener = Lwt.wait () in
+    interrupt_wakener := Some wakener;
+    (* Race between recv and interrupt *)
+    Lwt.pick
+      [
+        recv_impl conn;
+        ( interrupt_waiter >>= function
+          | Ok () -> Lwt.return (Error "Interrupted")
+          | Error _ -> Lwt.return (Error "Interrupted") );
+      ]
+    >>= fun result ->
+    (* Clear the wakener if we won the race *)
+    interrupt_wakener := None;
+    Lwt.return result
+
   let close conn =
     Lwt.catch
       (fun () ->
···
 
 let stats = { total = 0; matched = 0; last_seq = 0L; start = 0. }
 let json_mode = ref false
-let handle_sigint _ = interrupted := true
 
 let run config =
   let uri =
···
     Printf.printf "Press Ctrl+C to stop and show stats.\n\n%!"
   end;
   (* Use Lwt_unix signal handling for proper integration with Lwt *)
-  let _ = Lwt_unix.on_signal Sys.sigint (fun _ -> handle_sigint ()) in
+  let _ = Lwt_unix.on_signal Sys.sigint (fun _ -> trigger_interrupt ()) in
   stats.start <- Unix.gettimeofday ();
   Mirage_crypto_rng_unix.use_default ();
   (try run config with _ -> ());