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/gpu/gpu_pipeline.hpp

@@ -2,6 +2,18 @@
 
 #include <SDL3/SDL_gpu.h>
 
+// ============================================================================
+// PostFXUniforms — pushed to the fragment stage each frame via
+//   SDL_PushGPUFragmentUniformData(pass, 0, &uniforms, sizeof(PostFXUniforms))
+// MSL binding: constant PostFXUniforms& u [[buffer(0)]]
+// ============================================================================
+struct PostFXUniforms {
+    float vignette_strength;   // 0 = none, 1.5 = default subtle
+    float chroma_strength;     // 0 = off, 1 = full chromatic aberration
+    float scanline_strength;   // 0 = off, 1 = full scanlines
+    float time;                // accumulated seconds (for future animations)
+};
+
 // ============================================================================
 // GpuPipeline — Creates and owns the graphics pipelines used by the engine.
 //
@@ -9,6 +21,7 @@
 //                     Vertex layout: GpuVertex (pos float2, uv float2, col float4).
 //  postfx_pipeline_ : full-screen triangle, no vertex buffer, no blend.
 //                     Reads offscreen texture, writes to swapchain.
+//                     Accepts PostFXUniforms via fragment uniform buffer slot 0.
 // ============================================================================
 class GpuPipeline {
 public: