Boost: Enable / disable features independently on mobile and desktop

### Impacted plugin

Boost

### What

It would be nice to toggle modules independently for mobile and desktop users

### How

Some users have reported that some features (like Critical CSS) have different impacts on their Mobile and Desktop scores. In cases like these, it might be nice to enable the feature only for users on a specific device profile.

However, there is a big challenge here: Depending on what caching solutions a site uses, it may be impossible to serve different versions of a site on different devices without cache pollution and cross contamination.

This is an outline of an idea - here for discussion about whether or not it is feasible.

The issue names no files, tests, or entry points. Start by reviewing Boost's existing module settings and how device-specific caching is handled; define whether independent mobile and desktop toggles can avoid cache pollution, then document a concrete feasibility decision and implementation scope.

Written by the indexing model from the issue text.