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.
swaits
7a9820e495