Hot Reload via Atomic Pipeline Swap

When a route's steps change but its from: URI stays the same, hot reload compiles the new Pipeline and swaps atomically via ArcSwap. The Consumer is never stopped. Each in-flight Exchange completes against the Pipeline snapshot it entered — ArcSwap guarantees readers hold a stable reference for the lifetime of their access, so old and new Pipelines can coexist momentarily without corruption.

A simpler stop/recreate approach would work but adds unnecessary downtime and Consumer reconnection overhead. The atomic swap keeps the Consumer running and makes the reload effectively zero-downtime for the common case. The tradeoff is more complexity in the reload diffing logic (ReloadAction::Swap vs Restart). There is no explicit drain step — snapshot isolation via ArcSwap makes it unnecessary.