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/fuzz_targets/parse_command.rs

@@ -0,0 +1,14 @@
+//! `Command::from_bytes` must never panic on arbitrary input.
+//!
+//! Any accepted command must also respect the heapless payload bound.
+
+#![no_main]
+
+use donglora_protocol::{Command, MAX_PAYLOAD};
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    if let Some(Command::Transmit { payload, .. }) = Command::from_bytes(data) {
+        assert!(payload.len() <= MAX_PAYLOAD);
+    }
+});