feat: audio system with 15 procedurally generated sounds via soundgen

- AudioEngine with rodio (WAV playback, throttling, volume, toggle)
- 15 embedded sounds: jump, shoot, hit, explosion, death, pickup, descend,
  powerup, step, lava_bubble, acid_sizzle, water_splash, fire_crackle,
  ui_click, goblin_growl
- Sound events: shoot, hit, explosion (fireball), pickup, descend, powerup,
  jump, combat hit, ambient material sounds (lava/fire/acid/water)
- M key toggles audio in GPU modes
- Ambient sounds: scans 15-cell radius around player, plays throttled sounds
  for nearby lava/fire/acid/water
- AudioEngine gracefully degrades when no audio device available
- All 185 tests + 14 scenarios pass

src/audio/mod.rs

@@ -0,0 +1,149 @@
+use rodio::source::Source;
+use rodio::{OutputStream, Sink};
+use std::collections::HashMap;
+use std::io::Cursor;
+use std::time::Instant;
+
+pub struct AudioEngine {
+    _stream: Option<OutputStream>,
+    sink: Option<Sink>,
+    sounds: HashMap<&'static str, &'static [u8]>,
+    last_played: HashMap<&'static str, Instant>,
+    volume: f32,
+    enabled: bool,
+}
+
+impl AudioEngine {
+    pub fn new() -> Self {
+        let (stream, handle) = match OutputStream::try_default() {
+            Ok(s) => s,
+            Err(e) => {
+                eprintln!("Audio init failed: {}", e);
+                return Self::disabled();
+            }
+        };
+        let sink = match Sink::try_new(&handle) {
+            Ok(s) => s,
+            Err(e) => {
+                eprintln!("Audio sink init failed: {}", e);
+                return Self::disabled();
+            }
+        };
+
+        let mut sounds = HashMap::new();
+        sounds.insert(
+            "jump",
+            include_bytes!("../../assets/sounds/jump.wav") as &[u8],
+        );
+        sounds.insert("shoot", include_bytes!("../../assets/sounds/shoot.wav"));
+        sounds.insert("hit", include_bytes!("../../assets/sounds/hit.wav"));
+        sounds.insert(
+            "explosion",
+            include_bytes!("../../assets/sounds/explosion.wav"),
+        );
+        sounds.insert("death", include_bytes!("../../assets/sounds/death.wav"));
+        sounds.insert("pickup", include_bytes!("../../assets/sounds/pickup.wav"));
+        sounds.insert("descend", include_bytes!("../../assets/sounds/descend.wav"));
+        sounds.insert("powerup", include_bytes!("../../assets/sounds/powerup.wav"));
+        sounds.insert("step", include_bytes!("../../assets/sounds/step.wav"));
+        sounds.insert(
+            "lava_bubble",
+            include_bytes!("../../assets/sounds/lava_bubble.wav"),
+        );
+        sounds.insert(
+            "acid_sizzle",
+            include_bytes!("../../assets/sounds/acid_sizzle.wav"),
+        );
+        sounds.insert(
+            "water_splash",
+            include_bytes!("../../assets/sounds/water_splash.wav"),
+        );
+        sounds.insert(
+            "fire_crackle",
+            include_bytes!("../../assets/sounds/fire_crackle.wav"),
+        );
+        sounds.insert(
+            "ui_click",
+            include_bytes!("../../assets/sounds/ui_click.wav"),
+        );
+        sounds.insert(
+            "goblin_growl",
+            include_bytes!("../../assets/sounds/goblin_growl.wav"),
+        );
+
+        Self {
+            _stream: Some(stream),
+            sink: Some(sink),
+            sounds,
+            last_played: HashMap::new(),
+            volume: 0.5,
+            enabled: true,
+        }
+    }
+
+    fn disabled() -> Self {
+        Self {
+            _stream: None,
+            sink: None,
+            sounds: HashMap::new(),
+            last_played: HashMap::new(),
+            volume: 0.0,
+            enabled: false,
+        }
+    }
+
+    pub fn play(&mut self, name: &'static str) {
+        self.play_throttled(name, 50);
+    }
+
+    pub fn play_throttled(&mut self, name: &'static str, min_interval_ms: u64) {
+        if !self.enabled {
+            return;
+        }
+        let now = Instant::now();
+        if let Some(&last) = self.last_played.get(name) {
+            if now.duration_since(last).as_millis() < min_interval_ms as u128 {
+                return;
+            }
+        }
+        self.last_played.insert(name, now);
+
+        let data = match self.sounds.get(name) {
+            Some(d) => *d,
+            None => return,
+        };
+        let sink = match &self.sink {
+            Some(s) => s,
+            None => return,
+        };
+        if let Ok(decoder) = rodio::Decoder::new(Cursor::new(data.to_vec())) {
+            let source = decoder.amplify(self.volume).convert_samples::<f32>();
+            sink.append(source);
+        }
+    }
+
+    pub fn set_volume(&mut self, vol: f32) {
+        self.volume = vol.clamp(0.0, 1.0);
+    }
+
+    pub fn toggle(&mut self) {
+        self.enabled = !self.enabled;
+        if let Some(sink) = &self.sink {
+            if !self.enabled {
+                sink.pause();
+            } else {
+                sink.play();
+            }
+        }
+    }
+
+    pub fn is_enabled(&self) -> bool {
+        self.enabled
+    }
+}
+
+impl Default for AudioEngine {
+    fn default() -> Self {
+        Self::new()
+    }
+}