addon.node : support cancelling transcription via AbortSignal

[buxuku]

Once whisper() starts in addon.node, there's no way to stop it. A long movie/audio file can run for tens of minutes, and in a Node/Electron app the only way for the user to cancel is to kill the process. There's no AbortSignal (or equivalent) on the JS side, even though whisper.cpp already has encoder_begin_callback / abort_callback for this.

This adds an optional params.signal that takes a standard AbortSignal. On abort, the addon sets a shared flag that those existing callbacks poll, so computation stops without touching the core library. The promise still resolves (it doesn't reject): result.cancelled is true and result.transcription keeps whatever segments finished before the abort. An already-aborted signal cancels immediately with no segments. Omit signal and behavior is unchanged — existing callers aren't affected.

I hit this in SmartSub: users start a long transcription and then want to cancel from the UI, but the addon had no way to stop mid-flight. Added cancel-example.js covering mid-flight abort, a pre-aborted signal, and a normal run without signal. Built the addon and ran node cancel-example.js on macOS — all three cases passed.