World folder is the source of truth

- actor/<id>/ holds actor.wasm, actor.json, source/, public/, data/, versions/
- the broker boots by scanning actor/; a changed actor.wasm is an upgrade,
  a rejected one stays in versions/ with a reason and the files come back
- /actor hides every data/ folder
- subscriptions live in the subscriber's actor.json
- cargo installs the SDK and points sausage-actor at it; no build limits
- PROTOCOL.md

cli/src/lib.rs

@@ -44,9 +44,11 @@ impl Host {
         let rx = broker.open_mailbox("broker");
         tokio::spawn(broker.clone().run_actor(rx));
         let rx = broker.open_mailbox(cargo_actor::ID);
-        tokio::spawn(Arc::new(cargo_actor::Cargo::new(world.clone())).run_actor(broker.clone(), rx));
+        tokio::spawn(Arc::new(cargo_actor::Cargo::new(world.clone())?).run_actor(broker.clone(), rx));
 
-        broker.start_all();
+        for change in broker.scan().await {
+            eprintln!("sausage: {change}");
+        }
         Ok(Host { world, broker, _lock: lock })
     }