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/CMakeLists.txt

@@ -6,3 +6,17 @@ add_imgui_app(chart_demo
     ${CMAKE_SOURCE_DIR}/functions/viz/heatmap.cpp
     # fps_overlay vive en fn_framework
 )
+
+# --- E2E tests (opt-in via -DFN_BUILD_TESTS=ON) ---
+if(FN_BUILD_TESTS)
+    add_imgui_app(chart_demo_tests
+        main.cpp
+        tests/chart_demo_tests.cpp
+        ${CMAKE_SOURCE_DIR}/functions/viz/line_plot.cpp
+        ${CMAKE_SOURCE_DIR}/functions/viz/scatter_plot.cpp
+        ${CMAKE_SOURCE_DIR}/functions/viz/bar_chart.cpp
+        ${CMAKE_SOURCE_DIR}/functions/viz/heatmap.cpp
+    )
+    # Excludes int main() from main.cpp so the test target provides its own.
+    target_compile_definitions(chart_demo_tests PRIVATE FN_TEST_BUILD)
+endif()