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>

shaders/postfx.vert

@@ -0,0 +1,10 @@
+#version 450
+layout(location=0) out vec2 v_uv;
+void main() {
+    // Full-screen triangle from vertex index (no vertex buffer needed)
+    // NDC/UV mapping matches the MSL version (SDL3 GPU normalizes Y-up on all backends)
+    vec2 positions[3] = vec2[3](vec2(-1.0,-1.0), vec2(3.0,-1.0), vec2(-1.0,3.0));
+    vec2 uvs[3]       = vec2[3](vec2(0.0, 1.0),  vec2(2.0, 1.0), vec2(0.0,-1.0));
+    gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
+    v_uv = uvs[gl_VertexIndex];
+}