feat(shaders): sistema de shaders runtime amb presets externs

- Afegir GpuShaderPreset i ShaderManager per carregar shaders des de data/shaders/
- Implementar preset ntsc-md-rainbows (2 passos: encode + decode MAME NTSC)
- Render loop multi-pass per shaders externs (targets intermedis R16G16B16A16_FLOAT)
- cycleShader(): cicla OFF→PostFX natius→shaders externs amb tecla X
- --shader <nom> per arrancar directament amb un preset extern
- CMake auto-descubreix i compila data/shaders/**/*.vert/.frag → .spv
- HUD F1 mostra 'Shader: <nom>' quan hi ha shader extern actiu

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

source/engine.hpp

@@ -19,8 +19,10 @@
 #include "gpu/gpu_ball_buffer.hpp"      // for GpuBallBuffer, BallGPUData
 #include "gpu/gpu_context.hpp"          // for GpuContext
 #include "gpu/gpu_pipeline.hpp"         // for GpuPipeline
+#include "gpu/gpu_shader_preset.hpp"    // for NTSCParams, GpuShaderPreset
 #include "gpu/gpu_sprite_batch.hpp"     // for GpuSpriteBatch
 #include "gpu/gpu_texture.hpp"          // for GpuTexture
+#include "gpu/shader_manager.hpp"       // for ShaderManager
 #include "input/input_handler.hpp"      // for InputHandler
 #include "scene/scene_manager.hpp"      // for SceneManager
 #include "shapes_mgr/shape_manager.hpp" // for ShapeManager
@@ -82,6 +84,11 @@ class Engine {
         void setInitialPostFX(int mode);
         void setPostFXParamOverrides(float vignette, float chroma);
 
+        // External shader presets (loaded from data/shaders/)
+        void cycleShader();
+        void setInitialShader(const std::string& name);
+        std::string getActiveShaderName() const;
+
         // Modo kiosko
         void setKioskMode(bool enabled) { kiosk_mode_ = enabled; }
         bool isKioskMode() const { return kiosk_mode_; }
@@ -130,6 +137,7 @@ class Engine {
         float getPostFXVignette()  const { return postfx_uniforms_.vignette_strength; }
         float getPostFXChroma()    const { return postfx_uniforms_.chroma_strength; }
         float getPostFXScanline()  const { return postfx_uniforms_.scanline_strength; }
+        bool  isExternalShaderActive() const { return active_shader_ != nullptr; }
 
     private:
         // === Componentes del sistema (Composición) ===
@@ -184,6 +192,12 @@ class Engine {
         float postfx_override_vignette_ = -1.f;  // -1 = sin override
         float postfx_override_chroma_   = -1.f;
 
+        // External shader system
+        std::unique_ptr<ShaderManager> shader_manager_;
+        GpuShaderPreset* active_shader_     = nullptr;  // null = native PostFX
+        int              active_shader_idx_ = -1;        // index into shader_manager_->names()
+        std::string      initial_shader_name_;           // set before initialize()
+
         // Sistema de zoom dinámico
         int current_window_zoom_ = DEFAULT_WINDOW_ZOOM;