`instances` for an application such as `/duck` is a hash of the server-relative resource for each instance and the associated state object:

    {
      "/duck/0000000000000000" => { :clients => {...}, ... },
      "/duck/0123456789ABCDEF" => { :clients => {...}, ... }
    }

The state object contains things like the timer that spawns the instance's tick messages, the play/pause/stop state, and the client list.

Iterating over a hash yields each entry as `[ key, value ]`, which gets splatted into `resource` (key) and `instance` (value) since the block provides two parameters. (If it only provided one, it would receive `[ resource, instance ]` arrays instead).