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.

Optionally monitors an :owner pid (passed through the child spec). When that owner goes :DOWN, the supervisor stops :normal — running terminate/2's flush, and not restarting (transient). This is the owner-monitored self-cleanup seam from .claude/guidelines/testable-supervision-trees.md §3: any caller gets deterministic teardown by passing owner: self(). With no owner (the production default) the document outlives its starter as before.