injectFieldsSupportingAddons

Walk FIELD_REGISTRY">FIELD_REGISTRY and return every field type that opted into addons (i.e., declared addons on its FieldTypeDefinition).

Useful for tooling — admin UIs surfacing "which fields can carry addons?", docs generators, MCP server cross-checks. Field types without an addons registration ("Tier 3" — toggle, checkbox, radio, slider) are excluded.

Must be called within an injection context.