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.

gradle/libs.versions.toml

@@ -18,6 +18,7 @@ activityCompose = "1.9.3"
 navigationCompose = "2.8.5"
 playServicesLocation = "21.3.0"
 coroutines = "1.9.0"
+usbSerial = "3.9.0"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
@@ -46,6 +47,7 @@ room-compiler = { group = "androidx.room", name = "room-compiler", version.ref =
 appcompat = { group = "androidx.appcompat", name = "appcompat", version.ref = "appcompat" }
 osmdroid = { group = "org.osmdroid", name = "osmdroid-android", version.ref = "osmdroid" }
 androidx-core-splashscreen = { group = "androidx.core", name = "core-splashscreen", version.ref = "splashscreen" }
+usb-serial-android = { group = "com.github.mik3y", name = "usb-serial-for-android", version.ref = "usbSerial" }
 junit = { group = "junit", name = "junit", version.ref = "junit" }
 kotlinx-coroutines-test = { group = "org.jetbrains.kotlinx", name = "kotlinx-coroutines-test", version.ref = "coroutines" }