test: cargo-fuzz targets on parser and framing

Four libFuzzer targets:
- parse_command: Command::from_bytes never panics; bounded payload.
- parse_radio_config: RadioConfig::from_bytes never panics.
- cobs_decoder_feed: CobsDecoder::feed is panic-free across arbitrary
  byte streams split into arbitrary chunks.
- cobs_roundtrip: structure-aware (via arbitrary crate) Response →
  cobs_encode_response → ucobs::decode yields identical raw bytes.

All four smoke-tested at 50k runs each with no findings. Intended CI
cadence is a weekly cron with a larger time budget; per-PR would be
expensive. Requires nightly + cargo-fuzz.

fuzz/.gitignore

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+target
+corpus
+artifacts
+coverage