Narwhal v0.6.0 – message broker for edge apps, now with channel persistence

hi again! i'm back with a new release of Narwhal, the lightweight message broker for edge apps i shared a while ago. the headline change in v0.6.0 is channel persistence: channels and their message history can now be durably stored on disk, so subscribers can replay missed messages after reconnecting. also swapped the async runtime from monoio to compio (still io_uring on Linux), and added first-class Prometheus metrics.