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/viewmodel/MqttViewModel.kt

@@ -9,12 +9,17 @@ import com.hawhamburg.micr0bu.data.mqtt.MqttPreferences
 import com.hawhamburg.micr0bu.data.mqtt.MqttPrefs
 import com.hawhamburg.micr0bu.data.mqtt.MqttRepository
 import com.hawhamburg.micr0bu.data.mqtt.ObuHardwarePreferences
+import com.hawhamburg.micr0bu.data.transport.EspLinkStatus
+import com.hawhamburg.micr0bu.data.transport.EspRxMode
 import com.hawhamburg.micr0bu.data.transport.ObuHardware
 import com.hawhamburg.micr0bu.data.transport.TransportType
 import com.hawhamburg.micr0bu.data.transport.UsbNetworkDetector
+import com.hawhamburg.micr0bu.data.transport.UsbSerialState
+import com.hawhamburg.micr0bu.data.transport.UsbSerialTransport
 import com.hawhamburg.micr0bu.domain.denm.DenmUseCase
 import com.hawhamburg.micr0bu.domain.usecase.UseCaseAlert
 import com.hawhamburg.micr0bu.domain.usecase.UseCaseType
+import com.hawhamburg.micr0bu.service.CamPinger
 import dagger.hilt.android.lifecycle.HiltViewModel
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
@@ -33,6 +38,8 @@ class MqttViewModel @Inject constructor(
     private val usbDetector: UsbNetworkDetector,
     private val camUseCaseRepository: CamUseCaseRepository,
     private val obuHardwarePrefs: ObuHardwarePreferences,
+    private val usbSerialTransport: UsbSerialTransport,
+    private val camPinger: CamPinger,
 ) : ViewModel() {
 
     // ── MQTT connection & messages ────────────────────────────────────────────
@@ -58,6 +65,19 @@ class MqttViewModel @Inject constructor(
         viewModelScope.launch { obuHardwarePrefs.setObuHardware(hardware) }
     }
 
+    /**
+     * ESP32-C5-only: whether received CAM traffic is processed or discarded — see [EspRxMode]'s
+     * KDoc for the important caveat that this doesn't actually disable the ESP32's receiver
+     * (it can't, without also breaking TX).
+     */
+    val espRxMode: StateFlow<EspRxMode> = obuHardwarePrefs.espRxModeFlow.stateIn(
+        viewModelScope, SharingStarted.Eagerly, EspRxMode.SEND_AND_RECEIVE,
+    )
+
+    fun setEspRxMode(mode: EspRxMode) {
+        viewModelScope.launch { obuHardwarePrefs.setEspRxMode(mode) }
+    }
+
     /** True when a 192.168.42.x USB-C tethering network is detected. */
     val usbConnected: StateFlow<Boolean> = usbDetector.usbNetwork
         .map { it != null }
@@ -66,6 +86,27 @@ class MqttViewModel @Inject constructor(
     /** Auto-detected OBU gateway IP on the USB interface. */
     val detectedObuIp: StateFlow<String?> = usbDetector.detectedGatewayIp
 
+    /** ESP32-C5 USB-serial link state (Phase 03) — see [UsbSerialTransport]. */
+    val usbSerialState: StateFlow<UsbSerialState> = usbSerialTransport.state
+
+    /** Latest firmware heartbeat + drop counters, null until the first STATUS frame arrives. */
+    val espLinkStatus: StateFlow<EspLinkStatus?> = usbSerialTransport.linkStatus
+
+    /** Non-zero means CAMs are being built and dropped — see [UsbSerialTransport.sendCamTx]. */
+    val camSendFailures: StateFlow<Int> = usbSerialTransport.consecutiveWriteFailures
+
+    // ── ESP32-C5 CAM pinger (manual bench test, Phase 03) ─────────────────────
+    // The ESP32-C5-path equivalent of the CiT One's manual DENM trigger below — a fixed-
+    // location 1 Hz CAM ping the user starts/stops from the V2X Monitor screen to verify the
+    // serial link + ESP32 TX/RX radio path independent of GNSS movement or trip recording.
+    // See CamPinger's KDoc.
+
+    val camPingerActive: StateFlow<Boolean> = camPinger.isActive
+    val camPingerSentCount: StateFlow<Int> = camPinger.sentCount
+
+    fun startCamPinger() = camPinger.start()
+    fun stopCamPinger() = camPinger.stop()
+
     // ── Prefs ─────────────────────────────────────────────────────────────────
 
     val mqttPrefs: StateFlow<MqttPrefs> = prefs.prefsFlow.stateIn(
@@ -158,6 +199,11 @@ class MqttViewModel @Inject constructor(
     fun connect()    = repo.connect()
     fun disconnect() = repo.disconnect()
 
+    /** Connect/disconnect the ESP32-C5 USB-serial link — separate from [connect]/[disconnect],
+     * which drive the CiT One's MQTT-over-USB-C/Wi-Fi path. See [ConnectionSetupScreen]. */
+    fun connectUsbSerial()    = usbSerialTransport.connect()
+    fun disconnectUsbSerial() = usbSerialTransport.disconnect()
+
     fun selectTopic(topic: String?) { _selectedTopic.value = topic }
     fun setAutoScroll(enabled: Boolean) { _autoScroll.value = enabled }
 
@@ -189,5 +235,12 @@ class MqttViewModel @Inject constructor(
     override fun onCleared() {
         super.onCleared()
         repo.disconnect()
+        camPinger.stop()
+        // Deliberately NOT usbSerialTransport.disconnect(): the transport is an app-scoped
+        // @Singleton also held by the foreground TripRecordingService (via CamTransmitLoop).
+        // Closing it here would tear the port down when the Activity goes away — e.g. swiping
+        // the app from Recents mid-recording — leaving the still-running service beaconing into
+        // a dead port. The port closes on explicit user Disconnect, on USB detach (handled
+        // inside the transport), or with the process. See UsbSerialTransport's "Ownership" KDoc.
     }
 }