Class PollingProxyTransfer<T>

Duplex transfer with polling that receives its fetcher from the previous node in the chain.

Capabilities: isInput, isOutput, isPollingSource, isPollingProxy, isPullable, isSubscribable, isTriggerable, isGate

Mechanics:

1. The constructor accepts config (interval, activated), but NOT a fetcher
2. setFetcher(fetcher) — sets the fetcher from the previous transfer, creates a Ticker via tickerFactory, starts polling if active
3. clearFetcher() — stops the Ticker, clears the fetcher
4. trigger() — calls fetcher(), notifies subscribers (if active)
5. pull() — calls fetcher() directly (if active and fetcher is set)
6. activate()/deactivate()/toggle() — control polling and state
7. destroy() — stops polling, clears the fetcher, unsubscribes subscribers

Error handling:

• If fetcher() throws an exception in trigger() or pull(), onError is called.
• With onError provided, the exception is suppressed (polling continues).
• Without onError, the exception is rethrown.
• The 'Fetcher is not defined' error is always rethrown (this is not a fetcher runtime error).

Difference from PollingSourceTransfer:

• Fetcher is NOT set in the constructor, but via setFetcher()
• Has isInput = true (duplex, can be an intermediate link)
• pull()/trigger() without a fetcher throws an Error
• pull()/trigger() without active — returns undefined / is ignored

Configuration (PollingProxyTransferConfig):

• interval: number — polling interval (ms)
• activated: boolean — initial polling state
• tickerFactory?: TickerFactory — custom ticker factory (default: RAFTicker.factory)
• onError?: ErrorHandler — fetcher error handler

Use cases:

• Intermediate polling node in a transfer chain
• Adapter between a pull-source and a subscribe-consumer
• Periodic reading from a buffer with emission into a stream