feat: Implement audio backend and APU debugging features

- Introduced a new audio backend system with SDL2 support, allowing for flexible audio management and improved performance.
- Added APU handshake tracking capabilities to monitor CPU-APU communication during audio program uploads, enhancing debugging and transfer diagnostics.
- Updated the Emulator class to integrate the new audio backend, ensuring compatibility with existing audio handling.
- Implemented an APU Debugger UI for real-time monitoring of handshake status, port activity, and transfer progress, improving user experience for debugging audio issues.
- Refactored audio-related code to streamline audio sample queuing and management, enhancing overall emulator performance.

src/app/core/features.h

@@ -14,7 +14,8 @@ class FeatureFlags {
  public:
   struct Flags {
     // Log instructions to the GUI debugger.
-    bool kLogInstructions = true;
+    // WARNING: Setting this to true causes SEVERE performance degradation
+    bool kLogInstructions = false;
 
     // Flag to enable the saving of all palettes to the Rom.
     bool kSaveAllPalettes = false;