test: proptest property testing + cargo-fuzz fuzzing for donglora-cobs

crates/donglora-cobs/fuzz/fuzz_targets/fuzz_roundtrip.rs

@@ -0,0 +1,30 @@
+//! Fuzz target: encode then decode arbitrary input. Output must match input.
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    if data.len() > 1024 {
+        return; // skip huge inputs to keep fuzzing fast
+    }
+
+    let max_enc = donglora_cobs::max_encoded_len(data.len()) + 1;
+    let mut enc_buf = vec![0u8; max_enc];
+
+    let enc_len = match donglora_cobs::encode(data, &mut enc_buf) {
+        Some(n) => n,
+        None => return, // encode failed (buffer too small shouldn't happen, but skip)
+    };
+    let encoded = &enc_buf[..enc_len];
+
+    // Encoded output must never contain 0x00.
+    assert!(!encoded.contains(&0x00), "encoded contains sentinel");
+
+    let mut dec_buf = vec![0u8; data.len() + 1];
+    let dec_len = donglora_cobs::decode(encoded, &mut dec_buf)
+        .expect("decode failed after successful encode");
+
+    assert_eq!(
+        &dec_buf[..dec_len], data,
+        "round-trip mismatch: encode→decode produced different output"
+    );
+});