test: miri-clean + mutation-hardened

Adds tests/mutation_killers.rs with six targeted tests derived from
cargo-mutants findings: cad_enabled true/false paths, Transmit
with-config boundary, CobsDecoder::reset behaviour, empty-frame
handling, and the cobs_encode_response success path.

Three mutants remain as provably equivalent — the justfile excludes
them by exact regex so any genuinely new mutant on the same line will
still surface. Updates test-miri to also run the integration tests
(proptest stays skipped because its file-persistence layer calls
getcwd). Gitignore picks up mutants.out / mutants.out.old.

cargo mutants: 89 caught, 9 unviable, 0 missed.

.gitignore

@@ -2,4 +2,6 @@
 /fuzz/target
 /fuzz/corpus
 /fuzz/artifacts
+/mutants.out
+/mutants.out.old
 Cargo.lock