Optional protocol for nodes with custom downstream activation patterns.

After coordination finalization (if any), apply_runnable/2 needs to activate downstream nodes. Most node types use the default pattern: find :flow successors and draw activation edges from the result fact.

Some node types have non-standard activation patterns:

• FanOut emits multiple facts, each activating all downstream nodes
• Condition/Conjunction activate via prepare_next_runnables after a satisfied match

Nodes that implement this protocol override the default activation logic. Nodes that do not implement it get the standard single-fact activation.

Return Value

Implementations must return {workflow, activation_events} where activation_events is a list of RunnableActivated events that were folded into the workflow. This allows apply_runnable/2 to capture activation edges in the event stream for replay correctness.

Example

defimpl Runic.Workflow.Activator, for: MyApp.CustomBroadcastNode do
  def activate_downstream(node, workflow, runnable) do
    # Custom multi-target activation logic
    Runic.Workflow.Private.activate_downstream_with_events(workflow, node, fact)
  end
end