One binary, many pipelines: a scenario is a config, not an app

Live-follow, historical fill, cold rebuild, and replica are the same two programs, selected entirely by a config file and a run verb.

• Two binaries, an ingest and a relay, never recompiled to change what they run.
• A verb picks the run shape; a YAML config picks the data path.
• Every scenario added this cycle needed zero new code.