0326 — Nested timeouts use context.WithTimeoutCause naming the budget that fired

Context. pkg/consumer/consumer.go has three nested context.WithTimeout call sites (WorkTimeout, AckMargin, ShutdownTimeout); when one fired, the caller saw a generic context.DeadlineExceeded with no hint of which budget expired.

Decision. All three sites use context.WithTimeoutCause, each naming its specific budget.

Consequences. Timeout errors identify the responsible budget. errors.Is classification is unaffected because Cause() is a separate accessor from Err().