FieldRegistry

Manages CPT registration, field group CRUD, and field definition CRUD for the custom fields system. Maintains the in-memory registry of which CPTs participate in the field system and reads from and writes to benecaster_field_groups and benecaster_fields tables. Core auto-registers benecaster_episode at boot. Add-ons call benecaster_register_field_cpt() (global wrapper) from a benecaster_boot callback. Registered as a singleton in the container.