feat: add lock-free SPSC ring buffer for audio streaming

AtomicU32-based storage avoids unsafe while maintaining SPSC guarantees.
Capacity: 4096 samples (~85ms at 48kHz). Exported from crate root.

src/lib.rs

@@ -28,7 +28,7 @@ pub use runtime::{
     AudioMixer, AudioOutput, ClientRuntime, EmulationState, FRAME_HEIGHT, FRAME_RGBA_BYTES,
     FRAME_WIDTH, FrameClock, FramePacer, HostConfig, InputProvider, JOYPAD_BUTTON_ORDER,
     JOYPAD_BUTTONS_COUNT, JoypadButton, JoypadButtons, NesRuntime, NoopClock, NullAudio, NullInput,
-    NullVideo, PacingClock, RuntimeError, RuntimeHostLoop, SAVE_STATE_VERSION, VideoMode,
+    NullVideo, PacingClock, RingBuffer, RuntimeError, RuntimeHostLoop, SAVE_STATE_VERSION, VideoMode,
     VideoOutput, button_pressed, set_button_pressed,
 };