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.cpp

@@ -417,6 +417,9 @@ void Engine::calculateDeltaTime() {
 }
 
 void Engine::update() {
+    // Accumulate time for PostFX uniforms
+    postfx_uniforms_.time += delta_time_;
+
     // Actualizar visibilidad del cursor (auto-ocultar tras inactividad)
     Mouse::updateCursorVisibility();
 
@@ -824,6 +827,7 @@ void Engine::render() {
         SDL_BindGPUGraphicsPipeline(pass2, gpu_pipeline_->postfxPipeline());
         SDL_GPUTextureSamplerBinding scene_tsb = {offscreen_tex_->texture(), offscreen_tex_->sampler()};
         SDL_BindGPUFragmentSamplers(pass2, 0, &scene_tsb, 1);
+        SDL_PushGPUFragmentUniformData(cmd, 0, &postfx_uniforms_, sizeof(PostFXUniforms));
         SDL_DrawGPUPrimitives(pass2, 3, 1, 0, 0);
 
         // UI overlay (alpha-blended, uses sprite pipeline)