Deferred functions for Stages

Extend `Stage` to have a queue of deferred functions that are executed in `process`. Callbacks can be added to this queue in `onInputValueChanged` to react to specific input changes.

I already have an implementation in another project that could easily be ported.

Start by locating the Stage implementation, its process method, and onInputValueChanged. Review how input changes are handled and how the existing implementation from the other project maps to this framework. Done means Stage supports deferred callbacks, queues callbacks for specific input changes, and executes the queue during process.

Written by the indexing model from the issue text.