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_context.hpp

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <SDL3/SDL_gpu.h>
+#include <SDL3/SDL_video.h>
+
+// ============================================================================
+// GpuContext — SDL_GPU device + swapchain wrapper
+// Replaces SDL_Renderer as the main rendering backend.
+// ============================================================================
+class GpuContext {
+public:
+    bool init(SDL_Window* window);
+    void destroy();
+
+    SDL_GPUDevice*       device()  const { return device_; }
+    SDL_Window*          window()  const { return window_; }
+    SDL_GPUTextureFormat swapchainFormat() const { return swapchain_format_; }
+
+    // Per-frame helpers
+    SDL_GPUCommandBuffer* acquireCommandBuffer();
+    // Returns nullptr if window is minimized (swapchain not available).
+    SDL_GPUTexture* acquireSwapchainTexture(SDL_GPUCommandBuffer* cmd_buf,
+                                             Uint32* out_w, Uint32* out_h);
+    void submit(SDL_GPUCommandBuffer* cmd_buf);
+
+    // VSync control (call after init)
+    bool setVSync(bool enabled);
+
+private:
+    SDL_GPUDevice*       device_           = nullptr;
+    SDL_Window*          window_           = nullptr;
+    SDL_GPUTextureFormat swapchain_format_ = SDL_GPU_TEXTUREFORMAT_INVALID;
+};