Use MPSC channel instead of Arc<Mutex<_>> for the audio graph

Instead of sharing the graph between the engine and RPC threads, let the engine own the graph and the RPC handlers send messages via an MPSC channel to fetch/mutate the graph.

No files or tests are named. Start by locating the audio engine's graph ownership and the JSON-RPC handlers, then trace how they currently share the graph; done means the engine owns it and RPC handlers communicate through an MPSC channel for graph fetches and mutations.

Written by the indexing model from the issue text.