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/demos_graph.cpp

@@ -6,6 +6,7 @@
 #include "viz/graph_force_layout.h"
 #include "viz/graph_force_layout_gpu.h"
 #include "viz/graph_layouts.h"
+#include "viz/graph_labels.h"
 #include "core/button.h"
 #include "core/tokens.h"
 
@@ -157,6 +158,10 @@ void demo_graph() {
     };
     static int s_apply_layout = 0; // se incrementa cuando hay que reaplicar
 
+    // Labels (issue 0049j). LabelPolicy controlable desde la UI.
+    static graph::LabelPolicy s_label_policy;
+    static bool s_labels_enabled        = true;
+
     if (s_needs_regen) {
         init_demo_types();
         generate_synthetic_graph(s_n_nodes, s_n_clusters,
@@ -238,6 +243,25 @@ void demo_graph() {
         ImGui::PopItemWidth();
         ImGui::SameLine();
         if (button("Apply layout", ButtonVariant::Subtle)) s_apply_layout++;
+
+        // --- Labels (issue 0049j) ---------------------------------------
+        ImGui::Checkbox("Labels", &s_labels_enabled);
+        ImGui::SameLine();
+        ImGui::PushItemWidth(140);
+        ImGui::SliderInt("Max visible", &s_label_policy.max_visible, 0, 1000);
+        ImGui::SameLine();
+        ImGui::SliderFloat("Font",      &s_label_policy.font_size,
+                           8.0f, 24.0f, "%.0f");
+        ImGui::SameLine();
+        ImGui::SliderFloat("Min px",    &s_label_policy.min_node_pixel_size,
+                           0.0f, 40.0f, "%.0f");
+        ImGui::PopItemWidth();
+        ImGui::SameLine();
+        ImGui::Checkbox("Selected", &s_label_policy.always_for_selected);
+        ImGui::SameLine();
+        ImGui::Checkbox("Hovered",  &s_label_policy.always_for_hovered);
+        ImGui::SameLine();
+        ImGui::Checkbox("Pinned",   &s_label_policy.always_for_pinned);
     }
 
     section("Stats");
@@ -359,6 +383,21 @@ void demo_graph() {
 
         graph_viewport("##graph_demo", s_graph, s_state, ImVec2(0, 460), cb);
 
+        // Labels overlay (issue 0049j). El callback formatea "#<idx>" en un
+        // buffer estatico por demo — apps reales (osint_graph) usaran un
+        // string pool de la BD origen.
+        if (s_labels_enabled) {
+            struct LblCtx { char buf[32]; };
+            static LblCtx s_lbl_ctx;
+            auto get_label = [](int idx, void* user) -> const char* {
+                auto* ctx = static_cast<LblCtx*>(user);
+                std::snprintf(ctx->buf, sizeof(ctx->buf), "#%d", idx);
+                return ctx->buf;
+            };
+            graph::graph_labels_draw(s_graph, s_state, s_label_policy,
+                                      get_label, &s_lbl_ctx);
+        }
+
         if (ImGui::BeginPopup("##graph_node_ctx")) {
             ImGui::Text("Node #%d", s_ctx_node);
             ImGui::Separator();