Add 2D endless runner game with pickups and coyote time

- Platformer endless runner: fixed player x, world scrolls left
- Logarithmic speed curve: initial + factor * ln(1 + t / time_scale)
- Enemies stomped from above; side/bottom contact kills player
- Procedural level generation with StdRng seeded from SystemTime
- Object pooling via Vec::retain + frontier-based generator
- Coyote time: grace window after leaving platform edge
- Data-driven pickup system with trait-based effects (ActiveEffect)
  - Invulnerability, JumpBoost, ScoreMultiplier — extend via config
- Score accumulates with per-effect multiplier; stomp bonuses scaled
- Game over screen with score, best score, restart prompt
- HUD: score, speed bar, active effect timer bars

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

src/enemy.rs

@@ -0,0 +1,34 @@
+#[derive(Clone)]
+pub struct Enemy {
+    pub x: f32,
+    pub y: f32,
+    pub width: f32,
+    pub height: f32,
+    pub alive: bool,
+}
+
+impl Enemy {
+    pub fn new(x: f32, y: f32, width: f32, height: f32) -> Self {
+        Self { x, y, width, height, alive: true }
+    }
+
+    pub fn scroll(&mut self, speed: f32, dt: f32) {
+        self.x -= speed * dt;
+    }
+
+    pub fn is_off_screen(&self) -> bool {
+        self.x + self.width < 0.0
+    }
+
+    pub fn top(&self) -> f32 {
+        self.y
+    }
+
+    pub fn bottom(&self) -> f32 {
+        self.y + self.height
+    }
+
+    pub fn right(&self) -> f32 {
+        self.x + self.width
+    }
+}