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/NOTES.md

@@ -0,0 +1,53 @@
+# OBU transmit firmware - Phase 2 (in progress: HLN-SV DENM beacon)
+
+Started. See `docs/04-transmit-setup.md` in the project root for build/flash
+steps and how to validate this against your own sniffer.
+
+## Toolchain: use a dedicated terminal (ESP-IDF 5.5.4)
+
+This project builds against the **global** ESP-IDF 5.5.4, NOT the 6.1 checkout
+that `obu-firmware` uses. Keep one terminal per toolchain and never export both
+in the same window - the second export inherits the first's
+`IDF_PYTHON_ENV_PATH` and then fails every dependency check (`click`,
+`esptool`, `cryptography`, ... "not met"). That is env-var bleed, not a broken
+install: do **not** run `install.bat` to "fix" it, that damages one of the two
+environments.
+
+| Terminal | Export | Project |
+|---|---|---|
+| Transmitter | `C:\Espressif\frameworks\esp-idf-v5.5.4\export.ps1` | this one |
+| OBU | `...\micrOBU_workspace\its-g5-receiver-firmware\esp-idf\export.ps1` | `obu-firmware` |
+
+If a terminal has already been used for the other IDF, clear the state first:
+
+```powershell
+$env:IDF_PYTHON_ENV_PATH = $null; $env:IDF_PATH = $null
+```
+
+Also note `build/` here was regenerated from scratch (its CMake cache still
+referenced an older source path under `micrOBU_workspace/v2x-obu-esp32c5/`,
+which makes `idf.py fullclean` refuse to run). If that error reappears, delete
+`build/` manually rather than fighting it.
+
+## CAM encoding
+
+`main/cam.c` IS compiled here (unlike `obu-firmware`'s copy, which is a
+reference only). It must stay bit-identical to `obu-firmware/main/cam.c` and
+the app's `CamUperCodec.kt` - all three encode the same wire format, and a
+one-bit divergence in any of them is invisible on the bench but wrong against
+real equipment. See the `CurvatureCalculationMode` comment in that file.
+
+Implements one profile so far: **HLN-SV** (aftermarket stationary recovery
+vehicle), causeCode 94 (stationaryVehicle), subCauseCode 0, active while the
+hazard-light GPIO is grounded. No location/alacarte containers.
+
+- `main/main.c` - entry point, the `phy_11p_set`/`phy_change_channel(5900,...)`
+  register hack, GPIO polling, TX loop
+- `main/denm.c` / `.h` - ASN.1 UPER encoding of a minimal DENM
+- `main/geonet.c` / `.h` - GeoNetworking Basic/Common/SHB headers + BTP-B
+- `main/dot11p.c` / `.h` - 802.11 OCB (QoS Data, broadcast) frame + LLC/SNAP
+
+Known gaps, tracked as TODOs in the source: no real GNSS (lat/long hardcoded
+0), no real time source (detectionTime/referenceTime hardcoded 0, decodes as
+2004-01-01), fixed (non-rotating) pseudonym MAC, SHB instead of GeoBroadcast
+(no multi-hop forwarding), unsecured (no IEEE 1609.2 signing).