Include some form of routing control so not all messages are delivered to all destinations

If other projects want to use this software, we might need to be able to limit the destinations that messages are delivered to.

If nothing else, other projects won't want messages from our CSP devices.

No files, tests, or entry points are named. Start by mapping how messages are currently delivered to destinations and clarify the routing policy needed for other projects, including whether CSP-device messages should be excluded; done should be a defined, testable way to limit delivery destinations.

Written by the indexing model from the issue text.