refactor(gpu): eliminar GPU compute boids (prevé crash macOS)

Elimina el kernel Metal O(N²) de boids en GPU que causava GPU timeout
a macOS amb >50K boles, arrossegant WindowServer fins al crash.

- Elimina gpu_boid_buffer.hpp/cpp (GpuBoidBuffer, BallComputeData, BoidParams)
- Elimina kBoidComputeMSL i kBallComputeVertMSL de gpu_pipeline
- Elimina boid_compute_pipeline_ i ball_compute_pipeline_
- Elimina use_gpu_boids_, boid_params_, ball_screen_uniforms_ de Engine
- Elimina syncAndExitGpuBoids() i tot el compute dispatch de render()
- Mode BOIDS ara usa sempre boid_manager_ (CPU, spatial hash O(N))
  i renderitza via gpu_ball_buffer_ instanced (mateix path que PHYSICS)

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

source/input/input_handler.cpp

@@ -105,7 +105,7 @@ bool InputHandler::processEvents(Engine& engine) {
 
                 // Toggle Modo Boids (comportamiento de enjambre)
                 case SDLK_B:
-                    // engine.toggleBoidsMode();
+                     engine.toggleBoidsMode();
                     break;
 
                 // Ciclar temas de color (movido de B a C)