0534 — The metrics domain's write door is pkg/metrics/producer

Context

pkg/consumer/metrics held the metrics event producer (a goroutine + a producer.Producer publishing abandoned/cleared GoRoutineEvents to __system.metrics) and the GoRoutineEvent message shape. That put the metrics domain’s write door inside the consumer tree: pkg/consumer/base imported all of producer transitively just to call Add/Remove, and the consumermetrics-vs-metrics name collision forced aliases everywhere. The package was also the tree’s only config without WithDefaults/Validate, had no ds nil-check, a nil-safe enqueue receiver, a reader-less Noop field, and unused ctx params on Add/Remove.

Decision

• GoRoutineEvent + NewGoRoutineEvent move to pkg/metrics vocabulary (goroutineevent.go) beside EventType and AbandonedRoutineKey.
• The producer moves to pkg/metrics/producer, named by the domain-layer pattern the controller half already follows: MetricsProducer in producer.go parallels MetricsController in controller.go, ProducerConfig in producer_config.go parallels ControllerConfig in controller_config.go. Inside, pkg/producer imports as iProducer (the i* collision alias); callers import metricsproducer (the alias convention).
• The arrow is the alert pattern — the domain’s write door owns the producer, consumers use it (AlertController is the template). Hiding base’s transitive producer dependency behind an interface seam was rejected: the coupling is real, the seam would be decoration.
• Fixed in the move: ProducerConfig gains WithDefaults/Validate; NewMetricsProducer nil-checks ds and resolves cfg; the nil-safe enqueue receiver, the Noop field, and the Add/Remove ctx params are deleted (abandonedEvents is nil-checked at NewBaseDefinition, so the receiver guard was dead).

Consequences

• pkg/consumer/metrics is deleted.
• Add/Remove are fire-and-forget queue writes with no ctx, matching what they always did.
• A domain needing a write door gets a producer/ layer package beside controller/, named by the same pattern.