feat(registry): index cpp/apps/* + e2e test infrastructure

registry/indexer.go ahora escanea <lang>/apps/*/app.md ademas de apps/ y
projects/*/apps/. cpp/apps/chart_demo y cpp/apps/shaders_lab pasan a estar
en registry.db con sus manifests.

Infraestructura de tests e2e (opt-in con -DFN_BUILD_TESTS=ON):
- vendor de Dear ImGui Test Engine (personal/open-source license).
- chart_demo_tests target con tests/chart_demo_tests.cpp.
- /e2e-cpp slash command para crear y ejecutar tests e2e.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/apps/chart_demo/main.cpp

@@ -7,6 +7,7 @@
 #include "viz/bar_chart.h"
 #include "viz/heatmap.h"
 #include "core/app_menubar.h"
+#include "core/logger.h"
 
 #include <cmath>
 #include <vector>
@@ -23,6 +24,7 @@ static bool data_initialized = false;
 
 static void init_data() {
     if (data_initialized) return;
+    fn_log::log_info("init_data: generando %d puntos sin/cos, 200 scatter, 10x10 heatmap", N);
     for (int i = 0; i < N; i++) {
         xs[i] = static_cast<float>(i) * 0.02f;
         ys_sin[i] = sinf(xs[i]);
@@ -37,9 +39,10 @@ static void init_data() {
         heat_data[i] = sinf(r * 0.5f) * cosf(c * 0.5f);
     }
     data_initialized = true;
+    fn_log::log_debug("init_data: ok");
 }
 
-static void render() {
+void render() {
     init_data();
 
     // MainMenuBar (solo Settings — chart_demo no tiene paneles toggleables)
@@ -76,6 +79,7 @@ static void render() {
     ImGui::End();
 }
 
+#ifndef FN_TEST_BUILD
 int main() {
     return fn::run_app({
         .title  = "fn_registry — Chart Demo",
@@ -83,6 +87,9 @@ int main() {
         .height = 900,
         .about  = {.name        = "chart demo",
                    .version     = "0.2.0",
-                   .description = "Demo de primitivos viz: line, scatter, bar, heatmap. AppConfig estandar + multi-viewport."}
+                   .description = "Demo de primitivos viz: line, scatter, bar, heatmap. AppConfig estandar + multi-viewport."},
+        .log    = {.file_path = "chart_demo.log",
+                   .level     = static_cast<int>(fn_log::Level::Debug)}
     }, render);
 }
+#endif