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.frag

@@ -0,0 +1,24 @@
+#version 450
+layout(location=0) in vec2 v_uv;
+layout(location=0) out vec4 out_color;
+layout(set=2, binding=0) uniform sampler2D scene;
+layout(set=3, binding=0) uniform PostFXUniforms {
+    float vignette_strength;
+    float chroma_strength;
+    float scanline_strength;
+    float time;
+} u;
+void main() {
+    float ca = u.chroma_strength * 0.005;
+    vec4 color;
+    color.r = texture(scene, v_uv + vec2( ca, 0.0)).r;
+    color.g = texture(scene, v_uv).g;
+    color.b = texture(scene, v_uv - vec2( ca, 0.0)).b;
+    color.a = texture(scene, v_uv).a;
+    float scan = 0.85 + 0.15 * sin(v_uv.y * 3.14159265 * 720.0);
+    color.rgb *= mix(1.0, scan, u.scanline_strength);
+    vec2 d = v_uv - vec2(0.5, 0.5);
+    float vignette = 1.0 - dot(d, d) * u.vignette_strength;
+    color.rgb *= clamp(vignette, 0.0, 1.0);
+    out_color = color;
+}