0619 — the Message type argument sits on Register, not on NewProducer / NewConsumer

Amends [0362].

Context

producer.NewProducer[OrderPlacedV1](ds, nil) names a type at the constructor, then Register(ctx, topicName) names the topic. [0362] made the constructor take the raw datastore, which is why the type argument stopped being inferable, and it has been written by hand at every constructor since. Go’s inference reads a call’s arguments only, never a later use, so the argument cannot disappear — but Go 1.27’s generic methods let it move. The question was where the type belongs: on the handle that owns a datastore and a config, or on the call that binds a type to a topic.

Decision

• Producer and Consumer are plain structs: NewProducer(ds, cfg) and NewConsumer(ds, cfg) take no type argument.
• Register carries it: func (p *Producer) Register[Message topic.Versioned](ctx, topicName) and func (c *Consumer) Register[Message topic.Versioned](ctx, group, topicName, bindings).
• The same rule runs down the producer stack and the compaction reader: a struct is generic only when it holds Message-typed state. ProducerController, ProducerDatastore, and CompactionController hold none, so they are plain structs built once per handle, and their methods carry [Message] — AppendMessage[Message topic.Versioned] reads topic.SchemaVersionOf[Message]() itself, so the controller’s schemaVersion field and constructor param are gone. ProducerInstance[Message], Batcher[Message] (its queue holds typed operations), ConsumerInstance[Message], BaseProvisioner[Message] (holds the handler), ProducerFunc, and InTransaction stay generic.
• A call with no Message-typed argument names the type at the call: heads.ListHeads[alert.Alert](ctx, topicId), controller.GetCompactionHeadInTx[Message](...).
• Every module’s go.mod (and go.work) moves to go 1.27.0; the quickstart prerequisite reads Go 1.27+.

Consequences

• One handle per process. The metrics producer and the two alert provisioners each held two Producer[T] fields for two payload types; MessageAdmin held two CompactionController[T] fields. Each is now one field with per-type method calls.
• The type argument reads beside the topic name — “this type, to that topic” — and the missing-SchemaVersion() compile error lands on the Register line, where the topic is.
• Rejected: a zero-value witness argument (NewProducer(ds, T{}, nil)) to make inference work — a decoy value in place of a visible type; keeping [T] on the constructor — the handle has no per-type state.