fix: the KCP io throttle reset itself every cycle, so it never throttled

The send and recv arms shared one counter, and an ICMP error on a connected
socket is reported once and then cleared — so the steady state is an
alternation: the send succeeds and clears the counter, the next recv reports
the error and finds the counter at 1, and logs. Every error still wrote a
line, at the ~100/s the previous commit set out to stop, while the
persistent-failure and recovery branches were unreachable.

Use one LogThrottle per direction instead of a hand-rolled counter. That
removes the shared state the bug lived in, drops a third throttling mechanism
in favour of the one already added, and leaves the surrounding `if let Err`
untouched rather than reshaping it into a match.

Also fix test_udp_uat's socket-error arm, the untreated twin of the punch_udp
site: it had no backoff at all, so a persistent error re-armed recv
immediately and spun the loop at CPU speed, one warn line per iteration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ExUfAkYbq8UC9pQCiLy8TQ

src/common.rs

@@ -2653,7 +2653,7 @@ pub async fn punch_udp(
         tokio::select! {
             _ = hbb_common::sleep(retry_interval.as_secs_f32()) => {
                 if tm.elapsed() > MAX_TIME {
-                    bail!("UDP punch is timed out, stop sending packets after {packets_sent:?} packets, {recv_errors} recv errors absorbed");
+                    bail!("UDP punch is timed out, stop sending packets after {:?} packets, {} recv errors absorbed", packets_sent, recv_errors);
                 }
                 let elapsed = last_send_time.elapsed();