Add callback to signal successful publish

Nobody has claimed this yet.

On several occasions, I have found myself wanting to know exactly when a particular call to `Topic.Publish` has written the message to the wire. This is either because of some business logic (e.g. wait until peers have been signaled before doing something) or because I want to pool the `[]byte` argument in high-throughput applications.

From an API perspective, this would ideally take the form of a `PubOpt`, allowing us to write:

```go
err := t.Publish(ctx, data, WithCallback(func(m *Message) {
pool.Put(m.Data) // sync.Pool
})
```

The main difficulty I'm seeing is that messages are fanned out to a number of peers via buffered channels, so this would likely require some kind of refcounting scheme to ensure the callback is not called prematurely.

Is this a sensible ask? Any recommendations?

Start at Topic.Publish and the PubOpt API, then trace how messages are fanned out through buffered channels to peers. Determine how a callback can run only after the message is written to the wire for all relevant peers, using refcounting or an equivalent lifecycle mechanism. Done means callers can safely pool the []byte data without premature reuse.

Written by the indexing model from the issue text.