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-firmware/main/cam.c

@@ -1,3 +1,8 @@
+// NOT COMPILED - deliberately absent from main/CMakeLists.txt's SRCS. This firmware no longer
+// encodes CAM at all: the phone builds and UPER-encodes it and sends the bytes down serial_link,
+// and this side only GeoNetworking-wraps opaque payloads. The file is kept as the byte-exact
+// reference the Kotlin encoder (app CamUperCodec.kt) was ported from, so fixes must be applied
+// here too or the next person porting from it reintroduces the bug.
 #include "cam.h"
 #include <string.h>
 
@@ -108,8 +113,14 @@ int cam_encode(const cam_fields_t *f, uint8_t *buf, size_t buf_len)
     //            CurvatureConfidence ENUM 8 values -> 3 bits
     bw_put_bits(&bw, 1023 - (uint32_t)(-1023), 11); // curvatureValue: unavailable(1023)
     bw_put_bits(&bw, 7, 3);                         // curvatureConfidence: unavailable(7)
-    // CurvatureCalculationMode ENUM {yawRateUsed,yawRateNotUsed,unavailable} -> 2 bits
-    bw_put_bits(&bw, 2, 2);                 // unavailable
+    // CurvatureCalculationMode ENUM {yawRateUsed,yawRateNotUsed,unavailable, ...} - note the
+    // extension marker: UPER encodes an extensible ENUMERATED as an extension bit followed by
+    // the root-list index, so this is 1 + 2 = 3 bits, NOT 2. This file previously wrote only the
+    // 2-bit index, which shifted yawRate and the entire low-frequency container one bit early for
+    // any standards-compliant receiver. Harmless between this project's own encoder and decoder
+    // (both had the same error); wrong against every third-party station.
+    bw_put_bits(&bw, 0, 1);                 // extension bit: value is in the root list
+    bw_put_bits(&bw, 2, 2);                 // unavailable(2)
     // YawRate: YawRateValue(-32766..32767)->16 (offset from -32766),
     //          YawRateConfidence ENUM 8 values -> 3 bits
     bw_put_bits(&bw, 32767 - (uint32_t)(-32766), 16); // yawRateValue: unavailable(32767)