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/

dev/issues/README.md

@@ -62,7 +62,7 @@
 | [0049e](completed/0049e-graph-types-extended.md) | graph_types modelo extendido + EntityType/RelationType | completado | alta | feature | parte de 0049 |
 | [0049f](completed/0049f-graph-renderer-symbols.md) | Renderer extendido: shapes SDF, icon atlas, flechas, edge styles | completado | alta | feature | parte de 0049 |
 | [0049g](completed/0049g-graph-source-operations.md) | graph_sources: lector operations.db + abstraccion funcional | completado | alta | feature | parte de 0049 |
-| [0049h](0049h-graph-force-layout-gpu.md) | graph_force_layout_gpu: compute shader + spatial hash | pendiente | media-alta | feature | parte de 0049 |
+| [0049h](completed/0049h-graph-force-layout-gpu.md) | graph_force_layout_gpu: compute shader + spatial hash | completado | media-alta | feature | parte de 0049 |
 | [0049i](0049i-graph-layouts-static.md) | graph_layouts (radial/hierarchical/fixed) + viewport multi-select | pendiente | media | feature | parte de 0049 |
 | [0049j](0049j-graph-labels.md) | graph_labels: render etiquetas con LabelPolicy | pendiente | media | feature | parte de 0049 |
 | [0049k](0049k-graph-explorer-app.md) | App graph_explorer (proyecto osint_graph) — integracion final | pendiente | alta | feature | parte de 0049 |