cleanup: demote clean-disconnect warn, drop dead --low-latency flag

handle_peer's `bridge ended with error: ...` log fired at WARN every
time a viewer cleanly closed — but bridge can only end three ways
(peer-close, local-socket-close, cancellation), none of which are real
errors. Collapsed to INFO for both Ok and Err arms; the message itself
still carries any error detail.

Also removed the `--low-latency` CLI flag and its HostOpts field. It
was a placeholder for an unimplemented Phase-2/3 SRT transport, never
read anywhere, and was generating a persistent dead_code warning. If
SRT ever happens, the flag can come back fresh.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/host/mod.rs

@@ -147,13 +147,10 @@ async fn handle_peer(
 
     let bridge = tunnel::bridge(quic_send, quic_recv, tcp);
     tokio::select! {
-        res = bridge => {
-            if let Err(e) = res {
-                tracing::warn!(%remote, "bridge ended with error: {e:#}");
-            } else {
-                tracing::info!(%remote, "bridge closed cleanly");
-            }
-        }
+        res = bridge => match res {
+            Ok(()) => tracing::info!(%remote, "bridge closed cleanly"),
+            Err(e) => tracing::info!(%remote, "bridge ended: {e:#}"),
+        },
         _ = cancel.cancelled() => {
             tracing::info!(%remote, "cancellation during stream");
         }