Pub/sub on string channels with Action<object>.
Build a minimal event bus:
```csharp
class EventBus
{
private readonly Dictionary<string, Action<object>> handlers = new();
public void Subscribe(string topic, Action<object> handler)
{
handlers[topic] = handlers.TryGetValue(topic, out var existing)
? existing + handler : handler;
}
public void Publish(string topic, object payload)
{
if (handlers.TryGetValue(topic, out var h)) h(payload);
}
}
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