Prototype spread extension mechanism (test driver)

### What needs to get done

The spread extension sits between the spread test definition and the actual actions being executed, decoupling the test implementation from the test command handling in craft-application. Implement a script that handles prepare/restore actions and ad-hoc testing backends.

### Why it needs to get done

To exercise and validate the testing framework with a real project.

Start by locating the spread test definition and the craft-application test command handling to understand the extension boundary. Trace how prepare and restore actions are represented, then define the test driver’s completion criteria around handling those actions and supporting ad-hoc testing backends.

Written by the indexing model from the issue text.