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.

cpp/apps/primitives_gallery/CMakeLists.txt

@@ -73,6 +73,8 @@ add_imgui_app(primitives_gallery
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_layouts.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_viewport.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_viewport_selection.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_labels.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_labels_select.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/graph_spatial_hash.cpp
     # GL loader (Linux no-op, Windows wglGetProcAddress)
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_loader.cpp