Facade: split subsystem into one class per file, sync post code blocks to Java 25, add README mapping

02-structural/facade/AudioMixer.java

@@ -0,0 +1,9 @@
+package facade;
+
+/** AudioMixer — normalises audio tracks after conversion. */
+class AudioMixer {
+    static VideoFile fix(VideoFile result) {
+        System.out.println("  AudioMixer: fixing audio tracks");
+        return new VideoFile(result.getFilename());
+    }
+}