test: run mutants under both feature configurations

test-mutants now runs cargo-mutants twice — once with defaults (defmt
derives present) and once with --no-default-features (derives absent)
— so feature-gated paths are covered in both shapes. Same three
equivalent mutants are excluded in both runs.

justfile

@@ -67,15 +67,25 @@ test-miri:
 
 # mutation testing (requires cargo-mutants).
 #
+# Runs under both feature configurations so that feature-gated paths
+# (currently only the defmt::Format derives) are covered in both shapes.
 # Three mutants are excluded as provably equivalent — swapping the
 # operator does not change behaviour because a downstream check catches
 # the resulting case. See the regex strings for the specific sites.
-test-mutants:
+test-mutants: test-mutants-default test-mutants-no-default
+
+test-mutants-default:
     cargo mutants --no-shuffle --test-tool=nextest \
         -E 'replace > with >= in Command::from_bytes' \
         -E 'replace > with >= in CobsDecoder::feed' \
         -E 'replace < with <= in cobs_encode_response'
 
+test-mutants-no-default:
+    cargo mutants --no-shuffle --test-tool=nextest --no-default-features \
+        -E 'replace > with >= in Command::from_bytes' \
+        -E 'replace > with >= in CobsDecoder::feed' \
+        -E 'replace < with <= in cobs_encode_response'
+
 # format code
 fmt:
     cargo fmt