0426 — Producer split into pure factory plus Register; the stored lifecycleCtx dropped

Context. The producer carried identity in its constructor and held a stored lifecycle context, so construction could fail operationally and shutdown was a piece of producer state.

Decision. NewProducer(ds, cfg) is a pure build step; identity moved to Register(ctx, topic, version), which is stateless and returns the registered instance. The stored lifecycleCtx was dropped: shutdown is expressed by each produce call’s own ctx, and the error family around the stored lifetime (ErrShutdownRequested, DisableGracefulShutdown) was deleted. MetricEventProducer.Run self-registers and drains, replacing its Register.

Consequences. One factory can register many independent lives, and a produce call carries the same contract as any database call — bounded only by the ctx it is given, with no separate admission gate. The trade: a caller producing on a background context is no longer refused during app shutdown.