`measure_many` hangs with `current_thread` tokio runtime

[ThinkerDreamer]

MeasureManyStream::next() never returns when using tokio's current_thread runtime, even when pinging localhost.

Reproduction:

use std::{net::IpAddr, time::Duration};
use futures_util::StreamExt;
use massping::DualstackPinger;

#[tokio::main(flavor = "current_thread")]
async fn main() {
    let localhost: IpAddr = "127.0.0.1".parse().unwrap();
    let pinger = DualstackPinger::new().unwrap();
    let mut stream = pinger.measure_many([localhost].into_iter());
    
    // This hangs forever
    stream.next().await;
}

Works with multi_thread runtime, hangs with current_thread.

Possible cause: In pinger.rs, the background receive task uses try_recv() to process subscriptions, then blocks on recv().await. With a single-threaded runtime, when a new subscription is sent after the task is already blocking on recv(), it never gets processed - the ICMP reply arrives but gets dropped because the subscriber isn't registered yet.

It appears to be a race condition:

• Subscribe message is in the channel
• Background task is blocked on socket recv
• Reply arrives before Subscribe is processed
• Reply gets dropped

Suggested fix: Use tokio::select! to wait on both the subscription channel and socket recv simultaneously. Edit: This was not a feasible solution.