feat: USB OTG CDC-ACM with custom VID:PID, disconnect UX

Switch all ESP32-S3 boards from UsbSerialJtag to USB OTG CDC-ACM via
embassy-usb. The USB device stack is now shared code — boards only
provide the platform-specific embassy-usb Driver.

- Custom VID:PID 1209:5741 (open-source VID, "WA" PID)
- CDC-ACM class with proper device enumeration
- COBS+postcard protocol over CDC-ACM packet read/write
- Concurrent USB device loop + protocol loop via join()
- On disconnect: sends DisplayCommand::Reset to show "Waiting for host..."
- Justfile: espflash no longer uses --monitor (OTG steals PHY)
- Test script updated to detect by new VID:PID

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

src/display/task.rs

@@ -120,6 +120,13 @@ pub async fn display_task(
                     state.display_on = true;
                     render_and_flush(&mut display, &state).await;
                 }
+                DisplayCommand::Reset => {
+                    state.last_status = RadioStatus::default();
+                    state.rssi_history = [NO_SIGNAL; RSSI_HISTORY_LEN];
+                    state.rssi_count = 0;
+                    state.current_slot_rssi = NO_SIGNAL;
+                    render_and_flush(&mut display, &state).await;
+                }
             },
             Either3::Third(()) => {
                 // Timer tick: advance sparkline slot