0362 — NewMessageProducer takes the raw PostgresDatastore and builds its datastores internally
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Context. The producer constructor was asymmetric with NewMessageConsumer: callers had to build a producer datastore themselves just to hand it back, and labs only ever constructed one for that purpose.
Decision. NewMessageProducer[T](t, ds, cfg) takes the raw *datastore.PostgresDatastore plus the renamed MessageProducerConfig and constructs producerDatastore and topic.TopicDatastore internally, matching NewMessageConsumer. NewProducerDatastore went unexported. pkg/topic exported TopicDatastore/NewTopicDatastore rather than growing a one-off package-level check function.
Consequences. The Message type parameter is no longer inferable — call sites name it explicitly. Exporting the topic datastore was judged fine ahead of the public-API-to-root restructure. The topic identity later moved out of the constructor into Register(ctx, topic, version) (2026-08-08); the internal-datastore-construction shape stands.