Decide which features a user sees

Each product feature has an ID, a priority, whether it is behind an A/B test, the locations where it is allowed, and the features it conflicts with. For one user (an ID and a location), decide which features they see.

A feature is eligible when the user's location is allowed and either it has no A/B test or the user falls in the test group (even user IDs). A user can opt in to a feature, which skips the A/B check but not the location check, or opt out, which hides it. Then resolve conflicts: go through eligible features from highest priority down (ties by input order) and keep each one that doesn't conflict with a feature already kept; a conflict listed on either side counts. Return the kept features in input order.