Add experiment controls to Cortext settings

[priethor]

What

Adds an Experiments screen to Cortext settings. Admins can turn registered features on or off for the whole site. Changes save straight away, and quick clicks don't make the toggles flash or revert.

Why

We need somewhere to try unfinished features without presenting them as permanent settings.

How

• Registering experiments in PHP and storing their enabled state for the site.
• Restricting changes to administrators and queuing saves so an older response can't undo a newer click.

Testing Instructions

1. Register two temporary experiments through the cortext_experiments filter.
2. Open Cortext → Settings → Experiments as an administrator.
3. Toggle both experiments quickly, then reload the page and confirm the values stick.
4. Open the Experiments URL as a non-administrator and confirm the controls are replaced by a permission notice.
5. Remove the temporary experiments and confirm the empty state appears.

Screen recording

Grabacion.de.pantalla.2026-07-14.a.las.17.32.22.mov