fix(direct): validate A-role accepted connection, skip stale ones

The Acceptor's accept() on the shared signal endpoint can dequeue
a stale QUIC connection from a previous call that the Dialer has
already dropped. This results in "connection lost" errors when
media datagrams are sent — 100% drops on both sides.

Fix: after accepting a connection, check close_reason(). If the
connection is already closed, log a warning and re-accept. Also
verify max_datagram_size() is available before returning.

Additionally: emit transport details (remote addr, max_datagram,
close_reason) in the call_engine_starting debug event so stale
connection issues are visible in the user-facing debug log.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/src/lib.rs

@@ -588,8 +588,17 @@ async fn connect(
     }
 
     let app_clone = app.clone();
+    // Log transport details for debugging direct P2P media issues
+    let transport_info = pre_connected_transport.as_ref().map(|t| {
+        serde_json::json!({
+            "remote": t.remote_address().to_string(),
+            "max_datagram": t.max_datagram_size(),
+            "close_reason": t.connection().close_reason().map(|r| format!("{r:?}")),
+        })
+    });
     emit_call_debug(&app, "connect:call_engine_starting", serde_json::json!({
         "is_direct_p2p": is_direct_p2p_agreed,
+        "transport": transport_info,
     }));
     let app_for_engine = app.clone();
     match CallEngine::start(relay, room, alias, os_aec, quality, reuse_endpoint, pre_connected_transport, is_direct_p2p_agreed, app_for_engine, move |event_kind, message| {