Support native fft node #19

Parent: #4

Add renderer/native support for Elementary's fft node.

Docs: https://www.elementary.audio/docs/reference/fft

Scope:

• Confirm how FFT analysis events/data are surfaced by Elementary renderers.
• Add native event/resource support needed for el.fft(...) on iOS and Android.
• Ensure data is forwarded to JS using the React Native event path without excessive audio-thread work.
• Add example or test coverage proving FFT data can be consumed from React Native.

Acceptance criteria:

• el.fft can be used in a React Native graph without native/runtime errors.
• FFT output/events are delivered to JS in a documented shape.
• Event delivery is gated/efficient when there are no JS listeners.
• Works on both iOS and Android, or documents any platform limitation.