feat(superdough/audiocontext): make AudioContext injection optional

- Modify `setDefaultAudioContext()` to accept an optional `existingAudioCtx` parameter.
- If provided, it will use the existing context; otherwise, it creates a new one.
- This allows for better integration with external audio systems that manage their own AudioContext.

packages/superdough/audioContext.mjs

@@ -1,7 +1,7 @@
 let audioContext;
 
-export const setDefaultAudioContext = () => {
-  audioContext = new AudioContext();
+export const setDefaultAudioContext = (existingAudioCtx) => {
+  audioContext = existingAudioCtx ?? new AudioContext();
   return audioContext;
 };