Phase 03: ESP32-C5 serial link hardening + bench diagnostics

Enumeration:
- Merge the library's stock probe table instead of replacing it, so adding
  Espressif 0x303A/0x1001 doesn't drop every other supported device
- Select the ESP32-C5 by VID/PID rather than list position
- Log USB interface descriptors to distinguish CDC data from the JTAG interface
Lifecycle:
- Don't close the shared port in MqttViewModel.onCleared() - the foreground
  recording service outlives the ViewModel and would beacon into a dead port
- Handle ACTION_USB_DEVICE_DETACHED so the UI stops reporting a stale link
- Implement the STATUS heartbeat on both sides (1 Hz) plus a phone-side watchdog
- Surface write failures and firmware drop counters on the CAM Pinger card
Protocol:
- Assert DTR/RTS on open (unverified on hardware - see FLASHING.md step 5)
- Raise SERIAL_LINK_MAX_PAYLOAD 160 -> 512 on both sides; real third-party CAMs
  exceed 160 and were being silently dropped at the resync branch
- Move the enlarged buffers off task stacks; serialize send_frame with a mutex
Firmware and app must be updated together - a 512/160 mismatch fails silently.

app/src/main/java/com/hawhamburg/micr0bu/MainActivity.kt

@@ -83,6 +83,8 @@ class MainActivity : AppCompatActivity() {
             val showBatteryOptPrompt     by tripViewModel.showBatteryOptPrompt.collectAsState()
             val useCaseEnabledMap        by mqttViewModel.useCaseEnabledMap.collectAsState()
             val obuHardware              by mqttViewModel.obuHardware.collectAsState()
+            val usbSerialState           by mqttViewModel.usbSerialState.collectAsState()
+            val espRxMode                by mqttViewModel.espRxMode.collectAsState()
 
             MicrOBUTheme(darkTheme = state.darkTheme) {
                 val view = LocalView.current
@@ -135,6 +137,7 @@ class MainActivity : AppCompatActivity() {
                                 mqttConnectionState = mqttConnectionState,
                                 activeTransport = activeTransport,
                                 obuHardware = obuHardware,
+                                usbSerialState = usbSerialState,
                                 usbCableConnected = usbConnected,
                                 obuStationTypeWarning = obuStationTypeWarning,
                                 obuStationType = obuStationType,
@@ -250,6 +253,8 @@ class MainActivity : AppCompatActivity() {
                                 onMqttPrefsChange = mqttViewModel::updatePrefs,
                                 obuHardware       = obuHardware,
                                 onObuHardwareChange = mqttViewModel::setObuHardware,
+                                espRxMode         = espRxMode,
+                                onEspRxModeChange = mqttViewModel::setEspRxMode,
                                 onBack            = { navController.popBackStack() },
                             )
                         }
@@ -299,7 +304,13 @@ class MainActivity : AppCompatActivity() {
                             AboutSettingsScreen(onBack = { navController.popBackStack() })
                         }
                         composable(Screen.Connection.route) {
-                            ConnectionSetupScreen()
+                            // Must be the Activity-scoped instance (same one MainActivity holds
+                            // and MqttTopicViewerScreen is given below), not the default
+                            // hiltViewModel() — that would create a separate instance scoped to
+                            // this NavBackStackEntry, whose onCleared() (fired the moment you
+                            // navigate away) would disconnect the shared UsbSerialTransport out
+                            // from under every other screen still using it.
+                            ConnectionSetupScreen(viewModel = mqttViewModel)
                         }
                         composable(Screen.Map.route) {
                             MapScreen(gnss = state.gnss)