Manages the lifecycle of collaborative workflow document processes.

This GenServer coordinates a SharedDoc and PersistenceWriter for each collaborative workflow document. It starts both processes with proper dependencies, registers the SharedDoc with the :pg process group for cluster-wide coordination, and handles graceful shutdown by ensuring the SharedDoc flushes data to the PersistenceWriter before termination.

Uses a transient restart strategy, only restarting if the supervisor itself crashes, not when child processes exit normally. Monitors both child processes and stops itself if either child crashes.