fix: connect() no longer falls through from mux to USB serial

When a mux socket file exists, connect(None, timeout) and try_connect()
now commit to the mux — returning an error if unreachable instead of
silently falling through to USB serial. This prevents a race condition
where a client could steal the serial port from the mux during reconnect.

Callers with explicit port = Some(...) are unaffected.

Cargo.lock

@@ -44,7 +44,7 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
 [[package]]
 name = "donglora-client"
-version = "0.2.0"
+version = "0.2.1"
 dependencies = [
  "anyhow",
  "serialport",