Semi-coupled engine not compatible with Drupal 11.x

Overview

Now that 📌 Convert theme engines into services Needs work has been committed, using Canvas with the latest 11.x (For example, by following the DDEV installation steps) no longer works. When loading a canvas page, you get the Error 500: Service "semi_coupled" not found: the container inside "Drupal\Core\Theme\ThemeManager" is a smaller service locator that only knows about the "twig" service. error.

📌 Simplify canvas_stark Needs review talks about removing the theme engine all together, but I think we should get this working until thats committed.

Proposed resolution

Implement the changes from the CR → to turn the theme engine into a tagged service.

User interface changes

N/A