- Add optional downstream processor delegation via cdc-concurrent and cdc-parallel.
  - Default downstream runtime to cdc-concurrent, with cdc-parallel opt-in for Ruby 4 apps.
  - Add Rails example updates showing downstream processor wiring.
  - Extend benchmark support to measure direct downstream delegation overhead.
  - Document downstream runtime usage and benchmark interpretation.
  - Keep Rails integration, smoke tests, and benchmark tasks in sync with the new runtime model.