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-gui/src/app.rs

@@ -64,6 +64,8 @@ pub struct SessionTab {
     pub plot_view: plot_view::PlotViewState,
     pub view: View,
     pub auto_reconnect: bool,
+    pub dtr: bool,
+    pub rts: bool,
     pub send_input: String,
     pub send_mode: SendMode,
     pub append_newline: bool,
@@ -183,6 +185,10 @@ impl XserialApp {
     fn add_session_tab(&mut self, session_config: SessionConfig) {
         let history_limit = session_config.history_limit;
         let auto_reconnect = session_config.auto_reconnect;
+        let (dtr, rts) = match &session_config.transport {
+            TransportConfig::Serial { dtr, rts, .. } => (*dtr, *rts),
+            _ => (false, false),
+        };
         let handle = self.manager.create(session_config.clone());
         self.tabs.push(SessionTab {
             id: handle.id(),
@@ -194,6 +200,8 @@ impl XserialApp {
             plot_view: plot_view::PlotViewState::default(),
             view: View::Text,
             auto_reconnect,
+            dtr,
+            rts,
             send_input: String::new(),
             send_mode: SendMode::Text,
             append_newline: true,
@@ -920,6 +928,22 @@ fn render_session_controls(
                 tab.status = ConnectionStatus::Error(String::from("session not found"));
             }
         }
+        if matches!(tab.status, ConnectionStatus::Connected) {
+            let dtr_changed = ui.checkbox(&mut tab.dtr, "DTR").changed();
+            let rts_changed = ui.checkbox(&mut tab.rts, "RTS").changed();
+            if dtr_changed || rts_changed {
+                if let Some(handle) = manager.get(tab.id) {
+                    let dtr = tab.dtr;
+                    let rts = tab.rts;
+                    tokio::spawn(async move {
+                        if dtr_changed { let _ = handle.set_dtr(dtr).await; }
+                        if rts_changed { let _ = handle.set_rts(rts).await; }
+                    });
+                } else {
+                    tab.status = ConnectionStatus::Error(String::from("session not found"));
+                }
+            }
+        }
     });
     auto_reconnect_changed
 }
@@ -1062,6 +1086,8 @@ mod tests {
                 parity: SerialParity::None,
                 stop_bits: SerialStopBits::One,
                 flow_control: SerialFlowControl::None,
+            dtr: false,
+            rts: false,
             }),
             "Serial COM7"
         );

crates/xserial-gui/src/panels/config.rs

@@ -17,6 +17,8 @@ pub struct ConfigForm {
     pub serial_parity: SerialParity,
     pub serial_stop_bits: SerialStopBits,
     pub serial_flow_control: SerialFlowControl,
+    pub serial_dtr: bool,
+    pub serial_rts: bool,
     pub addr: String,
     pub udp_bind: String,
     pub udp_remote: String,
@@ -116,6 +118,8 @@ impl Default for ConfigForm {
             serial_parity: SerialParity::None,
             serial_stop_bits: SerialStopBits::One,
             serial_flow_control: SerialFlowControl::None,
+            serial_dtr: false,
+            serial_rts: false,
             addr: String::from("127.0.0.1:8080"),
             udp_bind: String::from("0.0.0.0:9000"),
             udp_remote: String::new(),
@@ -158,6 +162,14 @@ impl ConfigForm {
                 TransportConfig::Serial { flow_control, .. } => *flow_control,
                 _ => SerialFlowControl::None,
             },
+            serial_dtr: match &config.transport {
+                TransportConfig::Serial { dtr, .. } => *dtr,
+                _ => false,
+            },
+            serial_rts: match &config.transport {
+                TransportConfig::Serial { rts, .. } => *rts,
+                _ => false,
+            },
             addr: match &config.transport {
                 TransportConfig::Tcp { addr } => addr.clone(),
                 _ => String::from("127.0.0.1:8080"),
@@ -190,6 +202,8 @@ impl ConfigForm {
                     parity: self.serial_parity,
                     stop_bits: self.serial_stop_bits,
                     flow_control: self.serial_flow_control,
+                    dtr: self.serial_dtr,
+                    rts: self.serial_rts,
                 },
                 TransportChoice::Tcp => TransportConfig::Tcp {
                     addr: self.addr.clone(),
@@ -484,6 +498,8 @@ fn render_transport_section(ui: &mut Ui, form: &mut ConfigForm) {
                         );
                     });
             });
+            ui.checkbox(&mut form.serial_dtr, "DTR");
+            ui.checkbox(&mut form.serial_rts, "RTS");
         }
         TransportChoice::Tcp => {
             ui.horizontal(|ui| {