feat(audio): add audio_demo example with sine wave playback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/audio_demo/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "audio_demo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+voltex_audio.workspace = true

examples/audio_demo/src/main.rs

@@ -0,0 +1,38 @@
+use voltex_audio::{AudioClip, AudioSystem};
+
+fn generate_sine_clip(freq: f32, duration: f32, sample_rate: u32) -> AudioClip {
+    let num_samples = (sample_rate as f32 * duration) as usize;
+    let mut samples = Vec::with_capacity(num_samples);
+    for i in 0..num_samples {
+        let t = i as f32 / sample_rate as f32;
+        samples.push((t * freq * 2.0 * std::f32::consts::PI).sin() * 0.3);
+    }
+    AudioClip::new(samples, sample_rate, 1)
+}
+
+fn main() {
+    println!("=== Voltex Audio Demo ===");
+    println!("Generating 440Hz sine wave (2 seconds)...");
+
+    let clip = generate_sine_clip(440.0, 2.0, 44100);
+    let clip2 = generate_sine_clip(660.0, 1.5, 44100);
+
+    println!("Initializing audio system...");
+    let audio = match AudioSystem::new(vec![clip, clip2]) {
+        Ok(a) => a,
+        Err(e) => {
+            eprintln!("Failed to init audio: {}", e);
+            return;
+        }
+    };
+
+    println!("Playing 440Hz tone...");
+    audio.play(0, 0.5, false);
+    std::thread::sleep(std::time::Duration::from_secs(1));
+
+    println!("Playing 660Hz tone on top...");
+    audio.play(1, 0.3, false);
+    std::thread::sleep(std::time::Duration::from_secs(2));
+
+    println!("Done!");
+}