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.cpp

@@ -49,6 +49,10 @@ PFNGLFRAMEBUFFERTEXTUREPROC  fn_glFramebufferTexture  = nullptr;
 PFNGLBUFFERSUBDATAPROC       fn_glBufferSubData       = nullptr;
 PFNGLVERTEXATTRIBIPOINTERPROC fn_glVertexAttribIPointer = nullptr;
 PFNGLTEXBUFFERPROC           fn_glTexBuffer           = nullptr;
+PFNGLDISPATCHCOMPUTEPROC     fn_glDispatchCompute     = nullptr;
+PFNGLMEMORYBARRIERPROC       fn_glMemoryBarrier       = nullptr;
+PFNGLBINDBUFFERBASEPROC      fn_glBindBufferBase      = nullptr;
+PFNGLGETBUFFERSUBDATAPROC    fn_glGetBufferSubData    = nullptr;
 
 namespace fn::gfx {
 
@@ -104,6 +108,10 @@ bool gl_loader_init() {
     LOAD(glBufferSubData);
     LOAD(glVertexAttribIPointer);
     LOAD(glTexBuffer);
+    LOAD(glDispatchCompute);
+    LOAD(glMemoryBarrier);
+    LOAD(glBindBufferBase);
+    LOAD(glGetBufferSubData);
 
     #undef LOAD
     return true;