0510 — ConsumerType and its constants are demoted; NewConsumer defaults to cursor consumption

Context. consumer.ConsumerType with its CURSOR/LIFECYCLE constants and ConsumerConfig.Type made the consumption mode a config enum, exposing an internal mode switch as public vocabulary.

Decision. All three are demoted. consumer.NewConsumer defaults to cursor consumption; the lifecycle mode is reached via consumer.NewDeliveryConsumer directly.

Consequences. The constructor chosen, not a config field, says what kind of consumer you get — one fewer enum to document and keep stable, and no invalid mode/constructor combinations to validate.