Receive signed ITS messages and forward each at its declared length

Signed packets. A GeoNetworking Basic Header NextHeader of 2 means a
TS 103 097 (IEEE 1609.2) envelope follows, with the Common Header
inside it. gn_unwrap_its rejected all of these, and most real traffic is
signed: the 2026-08-17 capture holds 157 signed frames from 15 source
MACs against 2 unsecured stations. It now opens a COER-encoded
signedData, or a bare unsecuredData, and parses the inner packet as
before. The inner packet comes first inside tbsData, so the certificate
and signature are never parsed, and the signature is not verified - the
firmware has no trust store. Such messages reach the phone with the new
V2X_RX flags bit1, signed but not verified. The app reads only bit0 and
is unaffected until it learns the flag. Encrypted payloads, nested
signing and the legacy v1.2.1 envelope are still rejected. All 157
recorded signed frames have the layout this reads, in all three COER
length forms, and asn1tools decodes every envelope to the same inner
packet.

Payload bounds. Every frame recorded through the ESP32-C5's promiscuous
RX, about 15 000 of them, ends in 8 bytes that are not part of the
802.11 frame and not a valid FCS. obu-firmware reads frames through the
same API and took the rest of the frame as the message, so it forwarded
those 8 bytes to the phone after every message. UPER decoders stop where
the message ends, so nothing visibly broke, but the bytes cost serial
bandwidth and 8 bytes of the DENM's headroom, and they stayed attached
wherever raw payloads were stored or passed on. The payload is now
exactly what the Common Header's payload-length field declares, which is
also what separates a signed message from its signature.

A frame longer than main.c's 800-byte capture buffer is now reported as
truncated instead of being forwarded cut off, and counted as an oversize
drop through the new serial_link_note_oversize_drop, as it was when the
cut-off frame failed serial_link's size check.

Host tests in obu-firmware/test/host build the firmware sources
unmodified with MSYS2 gcc; `make` runs all three.
- test_chain: frames from the firmware's TX code checked byte by byte
  against EN 302 636-4-1 and parsed back, including hand-built signed
  frames, the payload-length rule, the RX trailer, and every truncation
  length against a no-access guard page. 1731 checks, 0 failures.
- test_replay and check_replay.py: all 15 145 recorded frames through
  gn_unwrap_its, cut to 800 bytes as on the board, and re-derived
  independently in Python with the envelope decoded by asn1tools. They
  agree on every record; 15 131 accepted, 157 of them signed. 11 043 of
  the 11 106 distinct messages re-encode byte-identically. The other 63
  fail the same way with the old 8 bytes put back, so the boundary is
  not the cause: 5 are our own CAMs from before the 2026-08-20
  yawRateConfidence fix, and the rest, from other stations, are a
  follow-up in TODO.md.
- fuzz_gn_unwrap: random edits of every recorded frame, each run against
  the guard page. 50 000 000 iterations, no crash.

obu-firmware/test/pcap_gn_tally.py tallies GeoNetworking header fields
per station over captures; it is how the other stations' lifetimes were
measured. TODO.md collects what is still open, including the on-air
check for this change: it builds on IDF 6.1 but has not been flashed.

obu-firmware/main/serial_link.h

@@ -37,10 +37,13 @@
 //       [0..1]   btp_dest_port  uint16 LE   2001 = CAM, 2002 = DENM (ETSI TS 103 248)
 //       [2]      rssi           int8        dBm, from the promiscuous RX metadata
 //       [3]      flags          uint8       bit0: geo area fields below are valid
+//                                           bit1: arrived signed (TS 103 097), signature NOT
+//                                                 verified. An app that tests only bit0 ignores it.
 //       [4..7]   geo_area_lat   int32 LE    1/10 microdegree, GeoBroadcast destination area
 //       [8..11]  geo_area_lon   int32 LE    1/10 microdegree
 //       [12..13] geo_area_dist  uint16 LE   Distance A, metres (relevance radius for a circle)
-//       [14..]   UPER message bytes
+//       [14..]   UPER message bytes - exactly the message. Before 2026-09-11 they were followed by
+//                the 8 bytes the chip's promiscuous RX appends (gn_unwrap.h, "Payload bounds").
 //
 //     All prefix fields are LITTLE-endian, matching this framing's own length field - note the
 //     GeoNetworking wire format they came from is big-endian, so gn_unwrap.c converts.
@@ -138,10 +141,11 @@ void serial_link_init(serial_link_cam_tx_cb_t on_cam_tx,
 // Sends a SERIAL_MSG_V2X_RX frame: the metadata prefix plus the UPER bytes gn_unwrap.c extracted
 // from an over-the-air frame. Pass has_geo_area=false and zeroes for the area fields when the
 // source frame carried no destination area (i.e. it was single-hop broadcast, not GeoBroadcast).
+// signed_unverified is gn_rx_t's flag of the same name; it sets bit1 of the prefix flags.
 // Returns true if the frame was written to the USB endpoint - not an end-to-end ack, the phone
 // may still drop it.
 bool serial_link_send_v2x_rx(uint16_t btp_dest_port, int8_t rssi,
-                              bool has_geo_area,
+                              bool has_geo_area, bool signed_unverified,
                               int32_t geo_area_lat_tenmicrodeg,
                               int32_t geo_area_lon_tenmicrodeg,
                               uint16_t geo_area_distance_a_m,
@@ -156,4 +160,9 @@ bool serial_link_send_status(uint8_t status);
 // otherwise indistinguishable, from the phone's side, from one that transmitted fine.
 void serial_link_note_tx_failure(void);
 
+// Counts an ITS message that cannot be forwarded because it is too large, in the same heartbeat
+// counter serial_link_send_v2x_rx() uses for its own size check. For main.c's rx_forward_task,
+// whose capture buffer is smaller than the largest frames on air.
+void serial_link_note_oversize_drop(uint16_t btp_dest_port);
+
 #endif