Restore batch buffering in WorkflowTagEngine while keeping paginated fan-out.
Group messages by workflow tag and name, flush tag mutations once, and buffer produced messages until batch processing finishes.
Process initial fan-outs immediately with paged ID reads, reuse page reads within a group, and only emit continuations when a next cursor exists.
Add tests for mixed batches, custom-id dispatch buffering, shared page reads, and storage interaction counts.

…rvice event envelope, and service executor envelope