An in-process loopback transport for hardware-free testing and examples.
[LoopbackTransport] implements the core Transport
trait against a shared in-memory [LoopbackBroker] instead of a network, so
examples, the simulators, and the cross-language conformance scenarios can
exercise the full publish/subscribe path with no broker process and no
hardware. Topic filters follow MQTT semantics, including the + single-level
and # multi-level wildcards.
Clone one broker into every transport that should share a namespace: a publish on any transport is delivered to every transport whose subscriptions match.
[Faulty] decorates any transport to inject send failures, so degraded-link
behavior such as store-and-forward retry can be tested deterministically.