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/dot11p.h

@@ -0,0 +1,31 @@
+#ifndef DOT11P_H
+#define DOT11P_H
+#include <stdint.h>
+#include <stddef.h>
+#include <stdbool.h>
+
+// Wraps a GeoNetworking-layer payload in an 802.11 OCB frame: QoS Data
+// (subtype 8, 26-byte header), matching real ITS-G5 hardware, broadcast, no
+// BSS (Addr1=Addr3=broadcast), LLC/SNAP with Ethertype 0x8947
+// (GeoNetworking's registered Ethertype). Output is ready to hand straight
+// to esp_wifi_80211_tx_custom() (tx_custom.c) - NOT esp_wifi_80211_tx(),
+// which rejects this frame type outright. `src_mac` is used as Addr2 - pass
+// the same 6 bytes you gave geonet_wrap_shb, since GN_ADDR's MID field is
+// defined to be this same link-layer address. Returns bytes written, or -1
+// if out buffer too small.
+//
+// History: this used to be downgraded to non-QoS Data (subtype 0) because
+// esp_wifi_80211_tx() rejects QoS Data ("unsupport QoS frame type" / esp_err
+// 258) and an attempted linker-override bypass (old main/wifi_patches.c)
+// didn't work. Restored to QoS Data now that main.c transmits via
+// esp_wifi_80211_tx_custom() instead, which bypasses that gate entirely
+// (see tx_custom.c) - so there's no longer a reason to deviate from the
+// real frame format.
+// qos=true  -> QoS Data (subtype 8, 26-byte header) for esp_wifi_80211_tx_custom()
+// qos=false -> plain Data (subtype 0, 24-byte header) which the STANDARD
+//              esp_wifi_80211_tx() accepts (used for the standard-TX isolation test)
+int dot11p_build_frame(const uint8_t *gn_payload, int gn_len,
+                        const uint8_t src_mac[6],
+                        uint8_t *out, size_t out_len, bool qos);
+
+#endif