Bitcoin Infrastructure Layer - Composition Root
This is the outermost layer that wires everything together. It serves as the composition root for dependency injection and the entry point for the entire Bitcoin implementation.
Graceful Shutdown
All background tasks listen to a shutdown signal via tokio::sync::watch.
The run() entry point waits for SIGINT (Ctrl+C) or SIGTERM, then triggers
an orderly shutdown with a configurable timeout.