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/main/cam.h

@@ -0,0 +1,35 @@
+#ifndef CAM_H
+#define CAM_H
+#include <stdint.h>
+#include <stddef.h>
+
+// Minimal CAM (Cooperative Awareness Message) per ETSI EN 302 637-2 v1.4.1
+// (CAM-PDU-Descriptions) + TS 102 894-2 v1.3.1 (CDD / ITS-Container), matching
+// the field set the working Rust reference (esp32-c_its-companion, feat/tx-cam,
+// src/applogic/cam_tx.rs) transmits:
+//   - ItsPduHeader (protocolVersion 2, messageID 2 = cam)
+//   - CoopAwareness { generationDeltaTime, camParameters }
+//   - CamParameters {
+//         basicContainer { stationType, referencePosition },
+//         highFrequencyContainer = basicVehicleContainerHighFrequency { ... },
+//         lowFrequencyContainer  = basicVehicleContainerLowFrequency  { ... }
+//     }
+// All vehicle-dynamics fields we don't measure are encoded as their ASN.1
+// "unavailable" value. Speed is a real 0 (correct for a stationary station).
+
+typedef struct {
+    uint32_t station_id;
+    uint8_t  station_type;            // StationType(0..255): 5 = passengerCar
+    uint16_t generation_delta_time;   // TimestampIts mod 65536 (ms); 0 until a real clock is wired
+    int32_t  latitude_tenmicrodeg;    // Latitude, 1/10 microdegree
+    int32_t  longitude_tenmicrodeg;   // Longitude, 1/10 microdegree
+    uint16_t speed_cm_s;              // SpeedValue, 0.01 m/s units (0 = stationary)
+    uint16_t heading_ddeg;            // HeadingValue, 0.1 deg units (0..3600), 3601 = unavailable
+    uint16_t vehicle_length_dm;       // VehicleLengthValue(1..1023), 10cm steps
+    uint8_t  vehicle_width_dm;        // VehicleWidth(1..62), 10cm steps
+} cam_fields_t;
+
+// Encodes the CAM as ASN.1 UPER. Returns bytes written, or -1 if buf too small.
+int cam_encode(const cam_fields_t *f, uint8_t *buf, size_t buf_len);
+
+#endif