Add MP3 download to UX test page.

Installs lamejs and adds an MP3 downloader that records the Motif synth output in real time via an output tap, then encodes and downloads as .mp3.

src/core/MotifEngine.ts

@@ -138,6 +138,25 @@ export class MotifEngine {
     }
   }
 
+  /**
+   * Connect an additional output tap node to the synthesis output.
+   * Useful for recording/export without changing the audible output.
+   */
+  connectOutput(node: AudioNode): void {
+    if (this.synthesisEngine) {
+      this.synthesisEngine.connectOutput(node);
+    }
+  }
+
+  /**
+   * Disconnect a previously connected output tap node.
+   */
+  disconnectOutput(node: AudioNode): void {
+    if (this.synthesisEngine) {
+      this.synthesisEngine.disconnectOutput(node);
+    }
+  }
+
   seek(progress: number): void {
     if (this.synthesisEngine) {
       this.synthesisEngine.seek(progress);