feat(gpu): migrar a SDL3_GPU amb 2-pass rendering i post-processat

- Infraestructura GPU: GpuContext, GpuPipeline, GpuSpriteBatch, GpuTexture
- Engine::render() migrat a 2-pass: sprites → offscreen R8G8B8A8 → swapchain + vignette
- UI/text via software renderer (SDL3_ttf) + upload com a textura overlay GPU
- CMakeLists.txt actualitzat per incloure subsistema gpu/

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

source/gpu/gpu_texture.hpp

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <SDL3/SDL_gpu.h>
+#include <SDL3/SDL_surface.h>
+#include <string>
+
+// ============================================================================
+// GpuTexture — SDL_GPU texture + sampler wrapper
+// Handles sprite textures, render targets, and the 1×1 white utility texture.
+// ============================================================================
+class GpuTexture {
+public:
+    GpuTexture()  = default;
+    ~GpuTexture() = default;
+
+    // Load from resource path (pack or disk) using stb_image.
+    bool fromFile(SDL_GPUDevice* device, const std::string& file_path);
+
+    // Upload pixel data from an SDL_Surface to a new GPU texture + sampler.
+    // Uses nearest-neighbor filter for sprite pixel-perfect look.
+    bool fromSurface(SDL_GPUDevice* device, SDL_Surface* surface, bool nearest = true);
+
+    // Create an offscreen render target (COLOR_TARGET | SAMPLER usage).
+    bool createRenderTarget(SDL_GPUDevice* device, int w, int h,
+                            SDL_GPUTextureFormat format);
+
+    // Create a 1×1 opaque white texture (used for untextured geometry).
+    bool createWhite(SDL_GPUDevice* device);
+
+    // Release GPU resources.
+    void destroy(SDL_GPUDevice* device);
+
+    SDL_GPUTexture* texture() const { return texture_; }
+    SDL_GPUSampler* sampler() const { return sampler_; }
+    int width()  const { return width_; }
+    int height() const { return height_; }
+    bool isValid() const { return texture_ != nullptr; }
+
+private:
+    bool uploadPixels(SDL_GPUDevice* device, const void* pixels,
+                      int w, int h, SDL_GPUTextureFormat format);
+    bool createSampler(SDL_GPUDevice* device, bool nearest);
+
+    SDL_GPUTexture* texture_ = nullptr;
+    SDL_GPUSampler* sampler_ = nullptr;
+    int width_  = 0;
+    int height_ = 0;
+};