2.0.0 Basic Demo broken: scene references a stale GUID → NullReferenceException on Play

Following the documented workflow (install via Package Manager git URL `https://github.com/inkle/ink-unity-integration.git#upm` → import the demo from *Packages > Ink Unity Integration > Demos* → open `Assets/Ink/Demos/Basic Demo/Basic Demo.unity` → Play) the demo does not run. The serialized story reference on `BasicInkExample` points to a GUID that exists nowhere in the package, so the field is null and Play immediately throws a `NullReferenceException`.

## Environment
- Unity 6000.5.9f1 (also reproducible on a fresh project)
- ink-unity-integration **2.0.0**, installed via UPM git URL (`#upm` branch, HEAD commit `73cc147` = "release refs/tags/2.0.0.")
- Imported the `Basic Demo.unitypackage` shipped inside the package with "Import All"

## Steps to reproduce
1. Create a new Unity project, install the package via the git URL above.
2. Project window → `Packages > Ink Unity Integration > Demos` → open `Basic Demo.unitypackage` → Import All.
3. Open `Assets/Ink/Demos/Basic Demo/Basic Demo.unity`.
4. Press Play.

Start by inspecting Assets/Ink/Demos/Basic Demo/Basic Demo.unity and the BasicInkExample component after importing the shipped Basic Demo.unitypackage. Trace its serialized story reference against the package assets, then verify that opening the scene and pressing Play no longer produces a NullReferenceException.

Written by the indexing model from the issue text.