Fix UPER encoding of CurvatureCalculationMode; verified on hardware

CurvatureCalculationMode is the one extensible ENUMERATED in CAM:
  ENUMERATED {yawRateUsed(0), yawRateNotUsed(1), unavailable(2), ...}
UPER encodes an extensible ENUMERATED as an extension bit followed by the root
index - 1 + 2 = 3 bits. All three of our encoders wrote only the 2-bit index,
shifting yawRate and the entire low-frequency container one bit early for any
standards-compliant receiver.

It went unnoticed because every end of this project shared the mistake: the
Kotlin codec was ported bit-for-bit from cam.c, so phone and ESP32 agreed
perfectly with each other and with nothing else. Confirmed against the ETSI
ASN.1 in the C-ITS-Parser checkout, where rasn marks this type - and only this
type - #[non_exhaustive].

Fixed in all three copies of the encoder (app CamUperCodec.kt,
obu-firmware/main/cam.c, obu-cam-transmistter/main/cam.c) plus the decoder,
which now rejects rather than misreads a set extension bit. Frame size is
unchanged at 43 bytes. Transmitter reflashed and the phone decodes its CAMs.

Also in this change:

- serial_link: skip send_frame entirely when no USB host is attached, and raise
  the tx mutex timeout above the worst-case hold. With the phone unplugged every
  write blocked its full timeout while holding the lock, so forwarded CAM_RX
  traffic starved the 1 Hz heartbeat - observed as "tx mutex timeout, dropping
  frame" on the console, and it would have tripped the phone's link watchdog.
  Verified gone on hardware.
- Log decoded and failed CAMs in CamUseCaseRepository. "The app shows nothing"
  had two indistinguishable causes; a silent `?: return` made this bug much
  harder to find than it needed to be.
- Remove the ESP32 send-only/send-and-receive toggle. Reception can't be
  disabled in firmware (raw TX only works while promiscuous), so it was an
  app-side filter pretending to be a radio control.
- V2X monitor follows the serial link state on the ESP32 path instead of MQTT,
  which is permanently disconnected there; CAM intake is gated on the link being
  up, and engine state is cleared when it drops.
- About screen: 0.5.0, Phase 03.
- Track obu-cam-transmistter, the bench CAM transmitter. Its cam.c is compiled
  (unlike obu-firmware's reference copy) and must stay bit-identical to the other
  two - this commit is what that coupling costs when it's broken.
- Document the two-toolchain split: this project builds on IDF 5.5.4, obu-firmware
  on the pinned 6.1. Exporting both in one shell fails confusingly.

app/src/main/java/com/hawhamburg/micr0bu/viewmodel/MqttViewModel.kt

@@ -10,7 +10,6 @@ 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
@@ -67,19 +66,6 @@ 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 }