Refactor Snes class and enhance Apu functionality

- Updated Snes constructor to initialize CPU callbacks directly, improving readability and maintainability.
- Removed unnecessary CpuCallbacks parameter from Cpu constructor, streamlining the class design.
- Added new methods in Apu for retrieving cycles, status, control, and handling DMA transfers, enhancing audio processing capabilities.
- Introduced unit tests for Apu to validate initialization, sample generation, and handshake timing, ensuring robust audio functionality.

src/app/emu/cpu/cpu.h

@@ -31,10 +31,12 @@ class InstructionEntry {
 
 class Cpu {
  public:
-  explicit Cpu(Memory& mem, CpuCallbacks& callbacks)
-      : memory(mem), callbacks_(callbacks) {}
+  explicit Cpu(Memory& mem) : memory(mem) {}
   void Reset(bool hard = false);
 
+  auto& callbacks() { return callbacks_; }
+  const auto& callbacks() const { return callbacks_; }
+
   void RunOpcode();
 
   void ExecuteInstruction(uint8_t opcode);
@@ -781,8 +783,8 @@ class Cpu {
   bool int_wanted_ = false;
   bool int_delay_ = false;
 
-  CpuCallbacks callbacks_;
   Memory& memory;
+  CpuCallbacks callbacks_;
 };
 
 }  // namespace emu