feat(gpu): afegir suport SPIRV (Vulkan) per Linux/Windows

- Nou: shaders/sprite.vert|frag, postfx.vert|frag, ball.vert (GLSL)
- Nou: cmake/spv_to_header.cmake — converteix .spv → uint8_t C header
- CMakeLists.txt: bloc non-Apple troba glslc, compila GLSL → SPIRV en
  build-time i genera headers embeguts a build/generated_shaders/
- gpu_context.cpp: MSL|METALLIB en Apple, SPIRV en la resta
- gpu_pipeline.cpp: createShaderSPIRV() + branques #ifdef __APPLE__
  per sprite/ball/postfx pipelines
- Corregeix crash a engine.cpp:821 (Windows/Linux) causat per pipelines
  null quan init() fallava en no trobar suport MSL

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

source/gpu/gpu_pipeline.hpp

@@ -48,6 +48,15 @@ private:
                                 Uint32 num_uniform_buffers,
                                 Uint32 num_storage_buffers = 0);
 
+    SDL_GPUShader* createShaderSPIRV(SDL_GPUDevice* device,
+                                     const uint8_t* spv_code,
+                                     size_t spv_size,
+                                     const char* entrypoint,
+                                     SDL_GPUShaderStage stage,
+                                     Uint32 num_samplers,
+                                     Uint32 num_uniform_buffers,
+                                     Uint32 num_storage_buffers = 0);
+
     SDL_GPUGraphicsPipeline* sprite_pipeline_ = nullptr;
     SDL_GPUGraphicsPipeline* ball_pipeline_   = nullptr;
     SDL_GPUGraphicsPipeline* postfx_pipeline_ = nullptr;