feat(viz): graph_force_layout_gpu compute + spatial hash (issue 0049h)

Layout force-directed en GPU usando 5 compute shaders 4.3 + spatial hash
grid 64x64. API simetrica con graph_force_layout (CPU) para que el consumer
pueda swappear sin cambios. atomicCompSwap loop para float-add portable.

- cpp/functions/viz/graph_force_layout_gpu.{h,cpp,md}: nuevo modulo
- cpp/functions/gfx/gl_loader: anade glDispatchCompute, glMemoryBarrier,
  glBindBufferBase, glGetBufferSubData (Windows wgl)
- cpp/tests/test_graph_force_layout_gpu.cpp: smoke + pinned + CPU vs GPU.
  Crea ventana GLFW oculta GL 4.3; SKIP si headless o sin compute.
- demos_graph: checkbox "GPU layout" para swappear CPU/GPU en runtime
- issue movido a dev/issues/completed/

cpp/functions/gfx/gl_loader.h

@@ -59,6 +59,11 @@
     extern PFNGLBUFFERSUBDATAPROC       fn_glBufferSubData;
     extern PFNGLVERTEXATTRIBIPOINTERPROC fn_glVertexAttribIPointer;
     extern PFNGLTEXBUFFERPROC           fn_glTexBuffer;
+    // Compute shaders + SSBOs — issue 0049h (graph_force_layout_gpu)
+    extern PFNGLDISPATCHCOMPUTEPROC     fn_glDispatchCompute;
+    extern PFNGLMEMORYBARRIERPROC       fn_glMemoryBarrier;
+    extern PFNGLBINDBUFFERBASEPROC      fn_glBindBufferBase;
+    extern PFNGLGETBUFFERSUBDATAPROC    fn_glGetBufferSubData;
 
     #define glAttachShader          fn_glAttachShader
     #define glBindBuffer            fn_glBindBuffer
@@ -107,6 +112,10 @@
     #define glBufferSubData         fn_glBufferSubData
     #define glVertexAttribIPointer  fn_glVertexAttribIPointer
     #define glTexBuffer             fn_glTexBuffer
+    #define glDispatchCompute       fn_glDispatchCompute
+    #define glMemoryBarrier         fn_glMemoryBarrier
+    #define glBindBufferBase        fn_glBindBufferBase
+    #define glGetBufferSubData      fn_glGetBufferSubData
 #else
     #define GL_GLEXT_PROTOTYPES
     #include <GL/gl.h>