feat(postfx): redisseny sistema PostFX (X/F5/F6, --postfx CLI)

- X cicla 4 efectes (Vinyeta/Scanlines/Cromàtica/Complet), sempre activa PostFX
- F5 fa toggle PostFX on/off mantenint l'efecte seleccionat
- F6 hereta el toggle d'integer scaling (abans F5)
- Arrencada per defecte sense postprocés (tot a 0)
- --postfx <vinyeta|scanlines|cromatica|complet> per activar des de CLI

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

source/engine.hpp

@@ -78,6 +78,8 @@ class Engine {
 
         // PostFX presets
         void handlePostFXCycle();
+        void handlePostFXToggle();
+        void setInitialPostFX(int mode);
 
         // Modo kiosko
         void setKioskMode(bool enabled) { kiosk_mode_ = enabled; }
@@ -170,8 +172,9 @@ class Engine {
         float delta_time_ = 0.0f;
 
         // PostFX uniforms (passed to GPU each frame)
-        PostFXUniforms postfx_uniforms_ = {1.5f, 0.0f, 0.0f, 0.0f};
+        PostFXUniforms postfx_uniforms_ = {0.0f, 0.0f, 0.0f, 0.0f};
         int postfx_effect_mode_ = 0;
+        bool postfx_enabled_ = false;
 
         // Sistema de zoom dinámico
         int current_window_zoom_ = DEFAULT_WINDOW_ZOOM;
@@ -244,6 +247,9 @@ class Engine {
         void updateShape();
         void generateShape();
 
+        // PostFX helper
+        void applyPostFXPreset(int mode);
+
         // GPU helpers
         bool loadGpuSpriteTexture(size_t index);   // Upload one sprite texture to GPU
         void recreateOffscreenTexture();            // Recreate when resolution changes