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_context.cpp

@@ -6,10 +6,12 @@
 bool GpuContext::init(SDL_Window* window) {
     window_ = window;
 
-    // Create GPU device — prefer Metal on macOS, Vulkan elsewhere
-    SDL_GPUShaderFormat preferred = SDL_GPU_SHADERFORMAT_MSL
-                                  | SDL_GPU_SHADERFORMAT_METALLIB
-                                  | SDL_GPU_SHADERFORMAT_SPIRV;
+    // Create GPU device: Metal on Apple, Vulkan elsewhere
+#ifdef __APPLE__
+    SDL_GPUShaderFormat preferred = SDL_GPU_SHADERFORMAT_MSL | SDL_GPU_SHADERFORMAT_METALLIB;
+#else
+    SDL_GPUShaderFormat preferred = SDL_GPU_SHADERFORMAT_SPIRV;
+#endif
     device_ = SDL_CreateGPUDevice(preferred, false, nullptr);
     if (!device_) {
         std::cerr << "GpuContext: SDL_CreateGPUDevice failed: " << SDL_GetError() << std::endl;