Fix APU buffering, timing, and audio pops

[nathsou]

Summary

• make the APU the sole audio producer and drain it from a synchronized ring buffer
• stop the Raylib callback and Web Audio consumer from advancing emulation
• preserve the last sample on underrun and prevent Web Audio queue overwrites
• fix triangle DAC popping, ultrasonic triangle handling, noise length/halt behavior, and mixer bounds
• distribute samples using a CPU-clock phase accumulator
• honor SMB's per-frame five-step $4017 sequencing and clear queued audio on state restore/mute
• keep the desktop frontend's audio queue primed without skipping normal game frames

Validation

• Native codegen/lib/*.c compile with Clang/C11
• node --check web/index.js
• node --check web/audio-worklet.js
• git diff --check

The full desktop link was not available in this environment because the checkout has no Raylib library. The wasm link was not available because the installed Clang has no wasm32 target.