Class ManualFlowTransfer<T>

Reactive stream with manual emission control via trigger().

Capabilities: isInput, isOutput, isPushable, isSubscribable, isTriggerable

Mechanics:

1. push(data) — writes the value (without notifying subscribers)
2. trigger() — notifies subscribers with the current value, clears _state
3. subscribe(handler) — subscribes to notifications
4. destroy() — unsubscribes subscribers, clears state

Difference from PushChannelTransfer:

• push() does NOT notify subscribers automatically
• To send data to subscribers, trigger() must be called
• Separates data writing from emission

Difference from ManualBufferTransfer:

• Uses subscribe() instead of pull()
• Notifies all subscribers on trigger(), not just one reader

Use cases:

• Synchronizing emission with an external event (e.g., requestAnimationFrame)
• Manual control of the data emission moment