test: three more cargo-fuzz targets + test-fuzz-long recipe

- command_roundtrip: structure-aware (via arbitrary crate) Command
  encode → parse → equal. Symmetric to cobs_roundtrip.
- cobs_decoder_multi_frame: chunked feeding with arbitrary split sizes;
  asserts bounded payload and that command count never exceeds sentinel
  count.
- cobs_chunk_invariant: same stream through two decoders (chunked vs
  all-at-once) must produce the same command sequence.

Adds test-fuzz-long recipe (600s per target, ~1h 10m total) for CI
cron / pre-release. All seven targets smoke-tested at 100k runs each
with zero findings.

justfile

@@ -44,6 +44,19 @@ test-fuzz:
     cd fuzz && cargo +nightly fuzz run parse_radio_config -- -max_total_time=60
     cd fuzz && cargo +nightly fuzz run cobs_decoder_feed -- -max_total_time=60
     cd fuzz && cargo +nightly fuzz run cobs_roundtrip -- -max_total_time=60
+    cd fuzz && cargo +nightly fuzz run command_roundtrip -- -max_total_time=60
+    cd fuzz && cargo +nightly fuzz run cobs_decoder_multi_frame -- -max_total_time=60
+    cd fuzz && cargo +nightly fuzz run cobs_chunk_invariant -- -max_total_time=60
+
+# long fuzz run: 10 minutes per target. Use for CI cron / pre-release.
+test-fuzz-long:
+    cd fuzz && cargo +nightly fuzz run parse_command -- -max_total_time=600
+    cd fuzz && cargo +nightly fuzz run parse_radio_config -- -max_total_time=600
+    cd fuzz && cargo +nightly fuzz run cobs_decoder_feed -- -max_total_time=600
+    cd fuzz && cargo +nightly fuzz run cobs_roundtrip -- -max_total_time=600
+    cd fuzz && cargo +nightly fuzz run command_roundtrip -- -max_total_time=600
+    cd fuzz && cargo +nightly fuzz run cobs_decoder_multi_frame -- -max_total_time=600
+    cd fuzz && cargo +nightly fuzz run cobs_chunk_invariant -- -max_total_time=600
 
 # run tests under miri (catches UB, requires nightly + miri component)
 # The proptest suite is skipped — its file-persistence layer calls getcwd