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/console.rs

@@ -115,7 +115,7 @@ impl Console {
         if built.body["type"] != "built" {
             bail!("build failed");
         }
-        let exists = self.broker.registry().actors.contains_key(actor);
+        let exists = self.broker.meta(actor).is_some_and(|m| m.version.is_some());
         let ty = if exists { "upgrade" } else { "spawn" };
         println!("· {ty} {actor} …");
         let body = json!({