Phase 03: real CAM UPER codec + ESP32-C5 TX/RX serial link

- Firmware: rewrite obu-firmware TX loop to be serial-driven (no on-chip timer), add promiscuous RX + GeoNetworking/BTP unwrap (gn_unwrap.c), add binary UART framing to the phone (serial_link.c/.h). Drop local cam_encode() - CAM is now built on the phone.
- Kotlin: byte-exact UPER CAM encoder/decoder ported from cam.c (BitWriter/BitReader/CamUperCodec), matching SerialFrame codec, real UsbSerialTransport (usb-serial-for-android), CamTransmitLoop (1Hz base rate, event/geofence boost, ESP32-C5-only), wired into CamUseCaseRepository for RX and TripRecordingService for TX.
- Add V2X message retention: persist all CAM (own+remote) to Room while recording, drop otherwise (DB v2 -> v3 migration).
- Add jitpack repo + usb-serial-for-android dependency.

Fixes: UsbSerialTransport now uses SerialInputOutputManager.start()/stop() (this lib version manages its own thread internally) instead of manual Runnable/Thread submission, which didn't compile.

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

@@ -82,6 +82,7 @@ class MainActivity : AppCompatActivity() {
             val tripServiceState         by tripViewModel.serviceState.collectAsState()
             val showBatteryOptPrompt     by tripViewModel.showBatteryOptPrompt.collectAsState()
             val useCaseEnabledMap        by mqttViewModel.useCaseEnabledMap.collectAsState()
+            val obuHardware              by mqttViewModel.obuHardware.collectAsState()
 
             MicrOBUTheme(darkTheme = state.darkTheme) {
                 val view = LocalView.current
@@ -133,6 +134,7 @@ class MainActivity : AppCompatActivity() {
                                 state = state,
                                 mqttConnectionState = mqttConnectionState,
                                 activeTransport = activeTransport,
+                                obuHardware = obuHardware,
                                 usbCableConnected = usbConnected,
                                 obuStationTypeWarning = obuStationTypeWarning,
                                 obuStationType = obuStationType,
@@ -145,6 +147,13 @@ class MainActivity : AppCompatActivity() {
                                     }
                                 },
                                 onNavigateToMap = { navController.navigate(Screen.Map.route) },
+                                onNavigateToRecord = {
+                                    navController.navigate(Screen.Record.route) {
+                                        popUpTo(Screen.Dashboard.route) { saveState = true }
+                                        launchSingleTop = true
+                                        restoreState = true
+                                    }
+                                },
                             )
                         }
                         composable(Screen.Sensors.route) {
@@ -239,6 +248,8 @@ class MainActivity : AppCompatActivity() {
                             ConnectionSettingsScreen(
                                 mqttPrefs         = mqttPrefs,
                                 onMqttPrefsChange = mqttViewModel::updatePrefs,
+                                obuHardware       = obuHardware,
+                                onObuHardwareChange = mqttViewModel::setObuHardware,
                                 onBack            = { navController.popBackStack() },
                             )
                         }