feat: scaffolding dag_engine_ui (issue 0095 step 2)

ImGui app shell:
- main.cpp con fn::run_app(cfg, render), panel Main placeholder.
- CMakeLists.txt + fn_framework + fn_table_viz opt-in.
- app.md frontmatter base.

Capas HTTP/WS/tabs en commits siguientes.

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

CMakeLists.txt

@@ -0,0 +1,14 @@
+add_imgui_app(dag_engine_ui
+    main.cpp
+)
+target_include_directories(dag_engine_ui PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
+
+# fn_table_viz: provides data_table::render(), viz_render, TQL engine, Lua, LLM.
+# Guard keeps the app compilable in builds where vendor/lua is absent.
+if(TARGET fn_table_viz)
+    target_link_libraries(dag_engine_ui PRIVATE fn_table_viz)
+endif()
+
+if(WIN32)
+    set_target_properties(dag_engine_ui PROPERTIES WIN32_EXECUTABLE TRUE)
+endif()