feat(postfx): afegir push constants i efectes chromatic aberration + scanlines

- PostFXUniforms struct (vignette_strength, chroma_strength, scanline_strength, time)
- Shader MSL actualitzat: aberració cromàtica RGB + scanlines sin-wave + vinyeta paramètrica
- Pipeline postfx declara num_uniform_buffers=1 (buffer(0) en MSL)
- Engine acumula temps i fa SDL_PushGPUFragmentUniformData cada frame
- Valors per defecte: vignette=1.5, chroma=0, scanlines=0 (comportament idèntic a l'anterior)

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

source/engine.hpp

@@ -163,6 +163,9 @@ class Engine {
         Uint64 last_frame_time_ = 0;
         float delta_time_ = 0.0f;
 
+        // PostFX uniforms (passed to GPU each frame)
+        PostFXUniforms postfx_uniforms_ = {1.5f, 0.0f, 0.0f, 0.0f};
+
         // Sistema de zoom dinámico
         int current_window_zoom_ = DEFAULT_WINDOW_ZOOM;