When merging gossips, also potentially apply/emit reachability change.

We should consider this:

- SWIM is distributed and spreads suspicion and unreachability
- [ ] TODO: check if we spread unreachability
- we gossip Members in high-level, they include reachability
- we know if a gossip is "ahead" and thus can pick the "more recent information about unreachability
- though that means we declared unreachability WITHOUT the SWIM declaring so locally, is this weird? Should we JUST rely on SWIMs to tell eachother the unreachability?

I would like to discuss this one some more, rather than just do what "we're used to" -- as our more smart failure detector does the spreading the info, so perhaps we should NOT merge this information on the high-level?

No files, tests, or entry points are named. Start by tracing SWIM's suspicion and unreachability propagation alongside high-level member gossip and reachability merging. Resolve whether newer unreachability data should be merged and emitted at the high level or left to SWIM, then document the decision and its observable behavior.

Written by the indexing model from the issue text.