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/geonet.h

@@ -0,0 +1,44 @@
+#ifndef GEONET_H
+#define GEONET_H
+#include <stdint.h>
+#include <stddef.h>
+
+// Wraps an ITS application payload (e.g. from denm_encode) with a minimal
+// GeoNetworking Basic Header + Common Header + Single-Hop-Broadcast
+// extended header (HeaderType=TSB(5), HeaderSubtype=SINGLE_HOP(0), per
+// ETSI EN 302 636-4-1 table 9), then prepends a BTP-B header addressed to
+// the DENM service port (2002).
+//
+// `mac` is the 6-byte pseudonym/link-layer address - pass the SAME address
+// you hand to dot11p_build_frame's src address, since GN_ADDR's MID field
+// (the last 6 bytes of the 8-byte GN_ADDR) is defined to BE that
+// link-layer address (EN 302 636-4-1 clause 9.5.1). `station_type` is the
+// 5-bit ITS-S type from the same clause (5 = passengerCar) and gets packed
+// into GN_ADDR alongside the address.
+//
+// `latitude_tenmicrodeg`/`longitude_tenmicrodeg` go into the Source Long
+// Position Vector (clause 9.5.2) as plain 32-bit signed big-endian fields -
+// NOT UPER bit-packed like the DENM payload's position fields, this is a
+// fixed-width binary protocol. Pass the SAME values you gave denm_encode's
+// eventPosition, so the GN-layer position and the DENM's own claimed
+// position agree.
+//
+// Deliberate simplification: real DENM dissemination normally uses
+// GeoBroadcast (GBC, HeaderType=4) so RSUs/OBUs can forward it across an
+// area - that needs a sequence number + geo-area fields this skeleton
+// doesn't build yet. Single-hop broadcast is simpler and is the
+// best-tested decode path in the receiver firmware you already have
+// working (same extended header shape as CAM). Fine for a single-vehicle
+// beacon; revisit if you need real multi-hop forwarding later.
+//
+// `btp_dest_port` is the BTP-B destination port for the service being carried
+// (ETSI TS 103 248): 2001 = CAM, 2002 = DENM, 2003 = MAPEM, 2004 = SPATEM, ...
+//
+// Returns bytes written, or -1 if out buffer too small.
+int geonet_wrap_shb(const uint8_t *its_payload, int its_len,
+                     const uint8_t mac[6], uint8_t station_type,
+                     int32_t latitude_tenmicrodeg, int32_t longitude_tenmicrodeg,
+                     uint16_t btp_dest_port,
+                     uint8_t *out, size_t out_len);
+
+#endif