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

crates/donglora-cobs/README.md

@@ -21,11 +21,21 @@ benchmark-optimized for streaming applications.
 ## Testing
 
 ```sh
+# Unit tests + property tests + interop
 cargo test -p donglora-cobs
+
+# Fuzz testing (requires cargo-fuzz: cargo install cargo-fuzz)
+cd crates/donglora-cobs
+cargo +nightly fuzz run fuzz_decode -- -max_total_time=60
+cargo +nightly fuzz run fuzz_roundtrip -- -max_total_time=60
 ```
 
 Tests include:
 - 18 canonical COBS vectors (Wikipedia / IEEE/ACM spec)
 - Round-trip verification for various payload sizes
-- 8 interop tests cross-validating against `corncobs` (dev-dependency)
+- 8 interop tests cross-validating against `corncobs`
 - 500 randomized round-trip payloads verified against `corncobs`
+- **5 proptest properties** (~1280 random inputs): round-trip, interop
+  encode/decode both directions, length bounds, decode-never-panics
+- **2 fuzz targets**: `fuzz_decode` (arbitrary bytes → must not panic)
+  and `fuzz_roundtrip` (encode→decode must match)