Add NES APU (Audio): Pulse, Triangle, Noise Channels

Document NES APU concepts in C.

Requirements:

• Define structs for PulseChannel, TriangleChannel, and NoiseChannel
• Implement a function that prints the duty cycle waveform for a pulse channel
• Implement a function that prints a simplified triangle wave pattern
• Implement a function that simulates the noise LFSR and prints a sample sequence
• Print how the frame counter steps work (4-step and 5-step modes)
• Show how pulse sweep units modify frequency over time
• Document how all five channels are mixed together into the final audio output
• Include a test that simulates one frame of audio generation and prints channel states