Class PushChannelTransfer<T>

Reactive channel with automatic emission to subscribers on push().

Capabilities: isInput, isOutput, isPushable, isSubscribable

Mechanics:

1. push(data) — writes the value to _state, notifies subscribers, clears _state
2. subscribe(handler) — subscribes to notifications
3. destroy() — unsubscribes all subscribers, clears state

Use cases:

• Fire-and-forget messaging between components
• Reactive events without state retention
• Analog of Observable with single emission per push()