Supervisor for Macula content-addressed storage system.

Manages the lifecycle of content system child processes: - macula_content_store: Local block and manifest storage - macula_content_transfer: Want/have/block exchange protocol

Supervision Strategy

Uses one_for_one strategy - if a child dies, only that child is restarted. This is appropriate because: - Store and Transfer can operate independently - Store failure shouldn't affect in-flight transfers - Transfer failure shouldn't corrupt storage

Integration

Add to macula_root.erl supervision tree:

   #{id => macula_content_system,
     start => {macula_content_system, start_link, [#{}]},
     type => supervisor}