v0.3.0: Compile size optimization & feature gates

What's Changed

Performance

• Replace heavy futures umbrella crate with lightweight BoxFuture type alias
• Remove unused tokio-stream dependency
• Use futures-util only behind nats feature gate
• Add release profile: opt-level=z, LTO, single codegen unit, symbol stripping

Architecture (from v0.2.0)

• Feature-gated optional modules: nats, encryption, cloudevents, routing
• Removed scaling.rs (domain coupling with Gateway/Box)
• Removed empty marker traits (WebhookSource, MetricsSource)
• Promoted build_subject/category_subject to trait default methods
• Cleaned NatsConfig redundant methods
• Deduplicated now_millis() utility

Stats

• 176 unit tests, 29 integration tests, 2 doc tests passing
• ~540 lines removed vs v0.1.0
• Minimal core compiles without nats/encryption/cloudevents/routing

Feature Flags

Feature	Default	Description
nats	✅	NATS JetStream provider
encryption	✅	AES-256-GCM payload encryption
cloudevents	✅	CloudEvents v1.0 conversion
routing	✅	Broker/Trigger event routing
full	—	All features