Reactive Portability

Three migration options for async stack:

• Keep Mutiny internally (lowest disruption, Quarkus-friendly, Spring via adapters).
• Migrate fully to Reactor (high disruption, no immediate benefit for Quarkus users).
• Add staged library neutrality (best long-term if done as adapter layer).

Recommended path is staged neutrality: keep current Mutiny contracts and add Reactor adapters where Spring needs them.

For core store/contract boundaries, prefer CompletionStage<T> for unary and reactive-streams Publisher<T> for streams.

The adaptation cost is significant because core execution classes are built around Mutiny operators (retry, transform, subscription, backpressure, failure handling). PipelineStepExecutor, QueueAsyncCoordinator, AwaitStepSupport, and telemetry paths need explicit semantics before adaptation.