Function discover_and_open_transport 

pub(crate) fn discover_and_open_transport(
    port: Option<&str>,
    baud: u32,
) -> Result<(String, EitherTransport), String>

Open a transport on the calling thread (must be main for BT).

This is synchronous — call from main before starting tokio. On macOS, Bluetooth RFCOMM callbacks require the main thread’s CFRunLoop, so transport discovery must happen before the tokio runtime is spawned on a dedicated thread.

Arguments

• port: explicit serial port path (e.g. /dev/cu.usbmodem*), or None for auto-detection of USB CDC or Bluetooth SPP.
• baud: baud rate (115200 for USB CDC, 9600 for BT SPP).

Returns

(port_path, transport) on success, or an error string.