0446 — ConsumerDatastore's phantom type parameter deleted

Context. ConsumerDatastore[Message any] carried a type parameter that appeared in no field, signature, or body — the datastore moves raw payload bytes, and unmarshalling happens above it.

Decision. The type parameter was deleted; the datastore is a plain type.

Consequences. Callers no longer name a type argument to instantiate a datastore whose behavior never depended on it, and two datastores over the same tables are now the same type instead of distinct instantiations. The signature stops claiming a type-dependence the code does not have.