ci: GitHub Actions workflow + drop test-fuzz-long

Eight parallel jobs on push/PR:

- test × 3 (default / no-default / all-features) via nextest + doc
- clippy × 3 feature permutations, -D warnings
- fmt
- doc (all-features, -D warnings on rustdoc)
- deny (cargo-deny action)
- miri (nightly, lib + vectors under default + no-default)
- fuzz (nightly, 60 s smoke on every target)
- mutants × 2 feature configs (same equivalence exclusions as local)

Swatinem/rust-cache on every cargo job; taiki-e/install-action for
nextest / cargo-fuzz / cargo-mutants. Uploads mutants.out on failure.

Drops test-fuzz-long from the justfile — 60 s per target in CI every
push plus the local proptest 1M-case suite already covers this.

justfile

@@ -48,16 +48,6 @@ test-fuzz:
     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
 # which miri does not support. Vectors and unit tests run clean.