feat(viz): graph_labels render con LabelPolicy + ImDrawList (issue 0049j)

graph_labels_draw pinta etiquetas de nodos sobre el FBO del graph_renderer
via ImDrawList. Politica configurable: always-on para selected/hovered/
pinned, top-N por size*(degree+1), culling por viewport AABB y
min_node_pixel_size. Cap duro = max_visible + |always_*|.

API:
- graph_labels_draw(graph, viewport_state, policy, cb, user)
- graph_labels_draw_at(...)  — variante con rect explicito
- graph_labels_select(...)   — helper puro testeable
- graph_compute_degrees(...) — O(E)

Splitting en dos TUs:
- graph_labels.cpp          — funciones draw (depende de ImGui)
- graph_labels_select.cpp   — helpers puros para tests sin ImGui

12 tests en test_graph_labels (culling, max_visible cap, min_pixel_size,
always_* gating por viewport, top-N por score, edge cases). Todos verdes.

Integrado en demos_graph con UI: toggle Labels, sliders Max visible /
Font / Min px, checkboxes Selected/Hovered/Pinned. Golden de
graph_viewport regenerado.

Cierra issue 0049j.

dev/issues/README.md

@@ -64,5 +64,5 @@
 | [0049g](completed/0049g-graph-source-operations.md) | graph_sources: lector operations.db + abstraccion funcional | completado | 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](completed/0049i-graph-layouts-static.md) | graph_layouts (radial/hierarchical/fixed) + viewport multi-select | completado | media | feature | parte de 0049 |
-| [0049j](0049j-graph-labels.md) | graph_labels: render etiquetas con LabelPolicy | pendiente | media | feature | parte de 0049 |
+| [0049j](completed/0049j-graph-labels.md) | graph_labels: render etiquetas con LabelPolicy | completado | 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 |