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/defines.hpp

@@ -51,7 +51,8 @@ constexpr float GRAVITY_CHANGE_LATERAL_MAX = 0.08f;  // Velocidad lateral máxim
 constexpr float BALL_SPAWN_MARGIN = 0.15f;  // Margen lateral para spawn (0.25 = 25% a cada lado)
 
 // Escenarios de número de pelotas (teclas 1-8)
-constexpr int BALL_COUNT_SCENARIOS[8] = {10, 50, 100, 500, 1000, 5000, 10000, 50000};
+// Fase 1 (instanced rendering): límit pràctic ~100K a 60fps (physics bound)
+constexpr int BALL_COUNT_SCENARIOS[8] = {10, 50, 100, 500, 1000, 5000, 10000, 100000};
 
 // Límites de escenario para modos automáticos (índices en BALL_COUNT_SCENARIOS)
 // BALL_COUNT_SCENARIOS = {10, 50, 100, 500, 1000, 5000, 10000, 50000}