feat: Add DTR/RTS control for serial ports with GUI toggles

- Assert DTR/RTS after serial port open to prevent wireless modules
  (HC-12/HC-15/Bluetooth bridges) from entering AT command mode after
  the unavoidable Linux kernel DTR pulse on every open() call.
- Add dtr/rts fields to TransportConfig::Serial (default: false).
- Add set_dtr()/set_rts() on SerialTransport, Connection, SessionHandle.
- Add SessionCmd::SetDtr/SetRts for live toggling via command channel.
- Add DTR/RTS checkboxes to GUI session controls (visible when connected)
  and to config form for persistence.
- Add test_plot_serial.c and test_text_serial.c — standalone C test
  tools for sending MixedTextPlot frames and plain text over serial.
- Update AGENTS.md with build prerequisites, architecture map, testing
  conventions, env vars, and known quirks.

crates/xserial-core/tests/pipeline.rs

@@ -688,6 +688,8 @@ async fn connection_transport_type_dispatch() {
         parity: SerialParity::None,
         stop_bits: SerialStopBits::One,
         flow_control: SerialFlowControl::None,
+            dtr: false,
+            rts: false,
     });
     let tcp = Connection::new(TransportConfig::Tcp {
         addr: "127.0.0.1:8080".into(),