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>

CMakeLists.txt

@@ -25,7 +25,7 @@ if (NOT SDL3_ttf_FOUND)
 endif()
 
 # Archivos fuente (excluir main_old.cpp)
-file(GLOB SOURCE_FILES source/*.cpp source/external/*.cpp source/boids_mgr/*.cpp source/input/*.cpp source/scene/*.cpp source/shapes/*.cpp source/shapes_mgr/*.cpp source/state/*.cpp source/themes/*.cpp source/text/*.cpp source/ui/*.cpp)
+file(GLOB SOURCE_FILES source/*.cpp source/external/*.cpp source/boids_mgr/*.cpp source/gpu/*.cpp source/input/*.cpp source/scene/*.cpp source/shapes/*.cpp source/shapes_mgr/*.cpp source/state/*.cpp source/themes/*.cpp source/text/*.cpp source/ui/*.cpp)
 list(REMOVE_ITEM SOURCE_FILES "${CMAKE_SOURCE_DIR}/source/main_old.cpp")
 
 # Comprobar si se encontraron archivos fuente