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()

cpp/apps/chart_demo/app.md

@@ -0,0 +1,60 @@
+---
+name: chart_demo
+lang: cpp
+domain: viz
+description: "Demo ImGui de primitivos viz del registry: line_plot, scatter_plot, bar_chart, heatmap. Cada chart en su propia tab del TabBar. Usado como showcase y como build gate de las funciones viz/."
+tags: [imgui, demo, charts, viz, showcase]
+uses_functions:
+  - line_plot_cpp_viz
+  - scatter_plot_cpp_viz
+  - bar_chart_cpp_viz
+  - heatmap_cpp_viz
+  # logger, app_menubar viven en fn_framework — no se listan aqui
+uses_types: []
+framework: "imgui"
+entry_point: "main.cpp"
+dir_path: "cpp/apps/chart_demo"
+repo_url: ""
+---
+
+## Que hace
+
+App de una sola ventana con cuatro tabs (Line / Scatter / Bar / Heatmap) que
+renderiza datos sinteticos para mostrar el aspecto y la API de los primitivos
+viz del registry. Sirve como:
+
+- **Showcase visual** de las funciones viz existentes — al añadir una nueva
+  primitiva, anadir su tab aqui es la forma natural de probar el binding.
+- **Build gate**: si una de las funciones rompe API, esta app deja de
+  compilar y lo cazamos sin tener que tocar `registry_dashboard` o
+  `graph_explorer`.
+
+## Estructura
+
+`main.cpp` (~93 lineas):
+
+- `init_data()` — genera arrays sinteticos una vez (estado modulo).
+- `render()` — DockSpaceOverViewport + TabBar con 4 tabs, cada una invoca
+  un primitivo del registry.
+- `main()` → `fn::run_app(...)` con AppConfig estandar (titulo, tamaño,
+  about, log).
+
+## Build
+
+```bash
+# Linux
+cd cpp && cmake -B build/linux -S . && cmake --build build/linux --target chart_demo
+
+# Windows (cross-compile)
+cd cpp && cmake -B build/windows -S . -DCMAKE_TOOLCHAIN_FILE=toolchains/mingw-w64.cmake \
+  && cmake --build build/windows --target chart_demo
+```
+
+## Decisiones
+
+- `viewports = true` (default de `fn::run_app`): las ventanas se pueden
+  arrastrar fuera del main window.
+- `init_gl_loader = false`: solo usa ImGui/ImPlot, sin gl* directo.
+- Sin persistencia propia (no abre BD).
+- `log: file_path = "chart_demo.log"` con nivel Debug — el `init_data`
+  emite info+debug para verificar que el logger funciona.

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

cpp/apps/chart_demo/tests/chart_demo_tests.cpp

@@ -0,0 +1,41 @@
+// E2E tests for chart_demo — Dear ImGui Test Engine.
+// Built only when -DFN_BUILD_TESTS=ON. The same main.cpp from chart_demo is
+// compiled here with FN_TEST_BUILD defined so its int main() is excluded and
+// only render() is reused.
+
+#include "app_base.h"
+#include "imgui.h"
+#include "imgui_te_engine.h"
+#include "imgui_te_context.h"
+
+void render();   // defined in chart_demo/main.cpp
+
+static void register_tests(ImGuiTestEngine* e) {
+    ImGuiTest* t = nullptr;
+
+    // Smoke test: the main window appears and is non-empty.
+    t = IM_REGISTER_TEST(e, "chart_demo", "smoke_window_visible");
+    t->TestFunc = [](ImGuiTestContext* ctx) {
+        ctx->SetRef("fn_registry \xe2\x80\x94 Chart Demo");   // em-dash
+        IM_CHECK(ctx->WindowInfo("").ID != 0);
+    };
+
+    // Cycle through all four tabs. Test engine fails the test if any tab item
+    // is not found or cannot be activated — that is our implicit assertion.
+    t = IM_REGISTER_TEST(e, "chart_demo", "tabs_cycle_all");
+    t->TestFunc = [](ImGuiTestContext* ctx) {
+        ctx->SetRef("fn_registry \xe2\x80\x94 Chart Demo");
+        ctx->ItemClick("##charts/Line Plot");
+        ctx->ItemClick("##charts/Scatter Plot");
+        ctx->ItemClick("##charts/Bar Chart");
+        ctx->ItemClick("##charts/Heatmap");
+    };
+}
+
+int main() {
+    fn::AppConfig cfg{};
+    cfg.title  = "chart_demo_tests";
+    cfg.width  = 1280;
+    cfg.height = 800;
+    return fn::run_app_test(cfg, render, register_tests);
+}