fix(lint): use Fmt module instead of Printf/Format (E205/E215/E216)
- Replace Printf.sprintf/printf with Fmt.str/pr in fuzz_squashfs, gen_corpus
- Replace failwith (Fmt.str ...) with Fmt.failwith in squashfs.ml (E215)
- Replace invalid_arg (Fmt.str ...) with Fmt.invalid_arg in squashfs_writer, tc (E216)
- Add fmt to gen_corpus dune deps for squashfs, srp, streaming-aead, tar, tc