Side-effect descriptors returned from init/1 and update/2.

An app's update/2 can return {new_model, cmd} to request side-effects. The runtime dispatches the cmd to a Task.Supervisor and re-enters the TEA loop; results arrive later as ordinary {:harlock_event, result} messages that update/2 handles like any other event.

def update(:fetch, model) do
  cmd = Cmd.from(fn -> HTTPoison.get!("https://example.com") end)
  {model, cmd}
end

def update({:ok, %{body: body}}, model), do: {%{model | body: body}, Cmd.none()}

Constructors:

• none/0 — no side-effect. Equivalent to returning just the model.
• from/1 — run a 0-arity function in a task; deliver its return value.
• batch/1 — dispatch a list of cmds concurrently; no ordering guarantee.
• map/2 — tag/transform the result of an inner cmd before delivery.

Task lifecycle: cmd tasks are supervised by Harlock.App.TaskSupervisor, itself a child of the app's supervisor positioned after Runtime. A Runtime exit terminates the task supervisor and all in-flight tasks (rest_for_one). A task crash is caught at the task body and delivered as {:cmd_error, reason}; it never propagates to the runtime.