Replay-Safe Workflow Logging

Workflow execution may replay the same code many times. Calling Logger directly would duplicate breadcrumbs, so workflow modules should use the journal-identified logging effect:

def run(input) do
  Continuum.log(:info, "starting order #{input.order_id}")
  # ...
end

Continuum.log/2 appends a workflow_log event carrying the command identity, level, and message. Only after that append succeeds does the live execution emit both Logger output and [:continuum, :workflow, :log] telemetry. Replay validates and consumes the event without emitting either output again.

Levels follow Logger (:debug through :emergency). Messages must be durable binaries and are capped at 16 KiB. Logs are workflow breadcrumbs, so avoid putting secrets in them; use an activity when the operation itself is an external side effect rather than observability.