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.

fuzz/Cargo.toml

@@ -49,3 +49,24 @@ path = "fuzz_targets/cobs_roundtrip.rs"
 test = false
 doc = false
 bench = false
+
+[[bin]]
+name = "command_roundtrip"
+path = "fuzz_targets/command_roundtrip.rs"
+test = false
+doc = false
+bench = false
+
+[[bin]]
+name = "cobs_decoder_multi_frame"
+path = "fuzz_targets/cobs_decoder_multi_frame.rs"
+test = false
+doc = false
+bench = false
+
+[[bin]]
+name = "cobs_chunk_invariant"
+path = "fuzz_targets/cobs_chunk_invariant.rs"
+test = false
+doc = false
+bench = false