Fix UPER encoding of CurvatureCalculationMode; verified on hardware

CurvatureCalculationMode is the one extensible ENUMERATED in CAM:
  ENUMERATED {yawRateUsed(0), yawRateNotUsed(1), unavailable(2), ...}
UPER encodes an extensible ENUMERATED as an extension bit followed by the root
index - 1 + 2 = 3 bits. All three of our encoders wrote only the 2-bit index,
shifting yawRate and the entire low-frequency container one bit early for any
standards-compliant receiver.

It went unnoticed because every end of this project shared the mistake: the
Kotlin codec was ported bit-for-bit from cam.c, so phone and ESP32 agreed
perfectly with each other and with nothing else. Confirmed against the ETSI
ASN.1 in the C-ITS-Parser checkout, where rasn marks this type - and only this
type - #[non_exhaustive].

Fixed in all three copies of the encoder (app CamUperCodec.kt,
obu-firmware/main/cam.c, obu-cam-transmistter/main/cam.c) plus the decoder,
which now rejects rather than misreads a set extension bit. Frame size is
unchanged at 43 bytes. Transmitter reflashed and the phone decodes its CAMs.

Also in this change:

- serial_link: skip send_frame entirely when no USB host is attached, and raise
  the tx mutex timeout above the worst-case hold. With the phone unplugged every
  write blocked its full timeout while holding the lock, so forwarded CAM_RX
  traffic starved the 1 Hz heartbeat - observed as "tx mutex timeout, dropping
  frame" on the console, and it would have tripped the phone's link watchdog.
  Verified gone on hardware.
- Log decoded and failed CAMs in CamUseCaseRepository. "The app shows nothing"
  had two indistinguishable causes; a silent `?: return` made this bug much
  harder to find than it needed to be.
- Remove the ESP32 send-only/send-and-receive toggle. Reception can't be
  disabled in firmware (raw TX only works while promiscuous), so it was an
  app-side filter pretending to be a radio control.
- V2X monitor follows the serial link state on the ESP32 path instead of MQTT,
  which is permanently disconnected there; CAM intake is gated on the link being
  up, and engine state is cleared when it drops.
- About screen: 0.5.0, Phase 03.
- Track obu-cam-transmistter, the bench CAM transmitter. Its cam.c is compiled
  (unlike obu-firmware's reference copy) and must stay bit-identical to the other
  two - this commit is what that coupling costs when it's broken.
- Document the two-toolchain split: this project builds on IDF 5.5.4, obu-firmware
  on the pinned 6.1. Exporting both in one shell fails confusingly.

obu-cam-transmistter/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