Changes

* A datapack registry json is loaded that refers to another registrable by id, e.g. "minecraft:desert"

* A datapack registry json is loaded that refers to another registrable by id, e.g. "minecraft:desert"

* A standalone reference holder is retrieved from the relevant registry via a get-or-create operation

* A standalone reference holder is retrieved from the relevant registry via a get-or-create operation

* When a datapack registry json is loaded and fully parsed, a holder for that json is get-or-created *and* the parsed value is bound to it

* When a datapack registry json is loaded and fully parsed, a holder for that json is get-or-created ''and'' the parsed value is bound to it

* When registries freeze, the get-or-create operation can no longer create new holders. If any reference holders in the registry are not fully bound at this time (with both a key and a value), an error is raised. This will occur if a datapack registry json refers by id to another datapack registry json that does not exist, and is how these references-by-id are validated.

* When registries freeze, the get-or-create operation can no longer create new holders. If any reference holders in the registry are not fully bound at this time (with both a key and a value), an error is raised. This will occur if a datapack registry json refers by id to another datapack registry json that does not exist, and is how these references-by-id are validated.

* Whenever tags load, each tag file's TagKey is bound to all reference holders referred to in that tag (this modifiers the holders in-place).

* Whenever tags load, each tag file's TagKey is bound to all reference holders referred to in that tag (this modifiers the holders in-place).

When datagenerating datapack registry jsons with reference holders, any reference holders *must* be created by the specific registry instances used by the RegistryOps/RegistryAccess used to datagen the jsons, or they will fail to serialize.

When datagenerating datapack registry jsons with reference holders, any reference holders ''must'' be created by the specific registry instances used by the RegistryOps/RegistryAccess used to datagen the jsons, or they will fail to serialize.

===== Intrusive Reference Holders =====

===== Intrusive Reference Holders =====