The user wants a full-screen flocking simulation of boids with the following characteristics:
1. Full-screen
2. Flocking behavior using separation, alignment, and cohesion
3. Boids drawn as oriented triangles
4. Smooth steering
5. Boids flee from the mouse cursor

I need to create a single index.html file that includes:
- Full-screen canvas
- CSS in a <style> tag
- JavaScript in a <script> tag
- Implementation of boids algorithm with separation, alignment, cohesion
- Mouse interaction (flee behavior)
- Triangles oriented based on velocity
- Smooth steering

Let me think about the implementation:

1. Boid class with:
   - Position (x, y)
   - Velocity (vx, vy)
   - Acceleration (ax, ay)
   - Max speed, max force
   - Methods: flock(boids), update(), render(), edges(), etc.

2. Flocking rules:
   - Separation: steer to avoid crowding neighbors
   - Alignment: steer towards average heading of neighbors
   - Cohesion: steer to move toward average position of neighbors

3. Additional rule:
   - Flee from mouse: steer away from mouse position

4. Main loop:
   - Update all boids
   - Draw all boids as oriented triangles

5. Canvas setup:
   - Full viewport
   - Handle resize

6. Triangle rendering:
   - Calculate angle from velocity
   - Draw triangle pointing in direction of velocity

I'll write this in a single HTML file.