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/gpu/gpu_pipeline.hpp

@@ -19,6 +19,9 @@ struct PostFXUniforms {
 //
 //  sprite_pipeline_ : textured quads, alpha blending.
 //                     Vertex layout: GpuVertex (pos float2, uv float2, col float4).
+//  ball_pipeline_   : instanced ball rendering, alpha blending.
+//                     Vertex layout: BallGPUData as per-instance data (input_rate=INSTANCE).
+//                     6 procedural vertices per instance (no index buffer).
 //  postfx_pipeline_ : full-screen triangle, no vertex buffer, no blend.
 //                     Reads offscreen texture, writes to swapchain.
 //                     Accepts PostFXUniforms via fragment uniform buffer slot 0.
@@ -33,7 +36,8 @@ public:
     void destroy(SDL_GPUDevice* device);
 
     SDL_GPUGraphicsPipeline* spritePipeline() const { return sprite_pipeline_; }
-    SDL_GPUGraphicsPipeline* postfxPipeline()  const { return postfx_pipeline_; }
+    SDL_GPUGraphicsPipeline* ballPipeline()   const { return ball_pipeline_; }
+    SDL_GPUGraphicsPipeline* postfxPipeline() const { return postfx_pipeline_; }
 
 private:
     SDL_GPUShader* createShader(SDL_GPUDevice* device,
@@ -41,8 +45,10 @@ private:
                                 const char* entrypoint,
                                 SDL_GPUShaderStage stage,
                                 Uint32 num_samplers,
-                                Uint32 num_uniform_buffers);
+                                Uint32 num_uniform_buffers,
+                                Uint32 num_storage_buffers = 0);
 
     SDL_GPUGraphicsPipeline* sprite_pipeline_ = nullptr;
+    SDL_GPUGraphicsPipeline* ball_pipeline_   = nullptr;
     SDL_GPUGraphicsPipeline* postfx_pipeline_ = nullptr;
 };