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.

obu-firmware/NOTES.md

@@ -0,0 +1,19 @@
+# OBU transmit firmware - Phase 2 (in progress: HLN-SV DENM beacon)
+
+Started. See `docs/04-transmit-setup.md` in the project root for build/flash
+steps and how to validate this against your own sniffer.
+
+Implements one profile so far: **HLN-SV** (aftermarket stationary recovery
+vehicle), causeCode 94 (stationaryVehicle), subCauseCode 0, active while the
+hazard-light GPIO is grounded. No location/alacarte containers.
+
+- `main/main.c` - entry point, the `phy_11p_set`/`phy_change_channel(5900,...)`
+  register hack, GPIO polling, TX loop
+- `main/denm.c` / `.h` - ASN.1 UPER encoding of a minimal DENM
+- `main/geonet.c` / `.h` - GeoNetworking Basic/Common/SHB headers + BTP-B
+- `main/dot11p.c` / `.h` - 802.11 OCB (QoS Data, broadcast) frame + LLC/SNAP
+
+Known gaps, tracked as TODOs in the source: no real GNSS (lat/long hardcoded
+0), no real time source (detectionTime/referenceTime hardcoded 0, decodes as
+2004-01-01), fixed (non-rotating) pseudonym MAC, SHB instead of GeoBroadcast
+(no multi-hop forwarding), unsecured (no IEEE 1609.2 signing).