feat: event infrastructure — payload, transport, NATS adapter

- EventPublisher now takes &DomainEvent (11 call sites + 3 impls updated)
- EventEnvelope + EventConsumer port in domain
- event-payload: serializable DomainEvent mirror with subject routing
- event-transport: generic Transport/MessageSource traits, publisher/consumer adapters
- adapters-nats: JetStream publish + durable pull consumer

crates/adapters/nats/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "adapters-nats"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+domain          = { workspace = true }
+event-transport = { workspace = true }
+async-nats      = { workspace = true }
+async-trait     = { workspace = true }
+futures         = { workspace = true }
+tokio           = { workspace = true }
+tracing         = { workspace = true }