proto: drop outcomes as effects (testability)

## Goal
Tests and the framework can't currently see *why* a message was dropped — only the logs show it, and logs aren't a test surface.

## Outcome
Drop outcomes observable as effects; logs unchanged. Pick up when the framework first needs per-drop causes.

## Needed
- [ ] Design (effect variant vs outcome type)
- [ ] Migrate the drop sites

## References
Consumer: #86

Start by reading consumer issue #86 and tracing the current message-drop sites in the repository. Decide whether drop causes belong in an effect variant or an outcome type, then migrate the drop sites so causes are observable as effects while logs remain unchanged.

Written by the indexing model from the issue text.