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.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

obu-firmware/main/serial_link.h

@@ -51,6 +51,13 @@
 // to SERIAL_LINK_MAX_PAYLOAD below.
 #define SERIAL_LINK_USB_BUF_SIZE  1024
 
+// Per-write block ceiling, and the mutex acquire timeout that must comfortably exceed the
+// worst case of one frame (4 writes: sync, head, payload, crc). Keep that relationship if you
+// change either number - a lock timeout below the max hold turns normal contention into
+// dropped frames, which is how the heartbeat was being starved by forwarded CAM_RX traffic.
+#define SERIAL_LINK_WRITE_TIMEOUT_MS   100
+#define SERIAL_LINK_TX_LOCK_TIMEOUT_MS 600
+
 // Max CAM payload this link will carry. MUST match SERIAL_LINK_MAX_PAYLOAD in the app's
 // SerialFrame.kt - a mismatch means every frame above the smaller of the two is rejected by that
 // side's "length exceeds max, resync" branch, silently.