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>

app.md

@@ -0,0 +1,42 @@
+---
+name: dag_engine_ui
+lang: cpp
+domain: tools
+description: "Frontend ImGui para dag_engine. Lista, lanza e inspecciona DAGs con live updates via WS."
+tags: [imgui]
+uses_functions:
+  # Uncomment when using data_table::render() — provided via fn_table_viz:
+  # - data_table_cpp_viz
+  # - viz_render_cpp_viz
+  # - compute_stage_cpp_core
+  # - compute_pipeline_cpp_core
+  # - compute_column_stats_cpp_core
+  # - auto_detect_type_cpp_core
+  # - tql_emit_cpp_core
+  # - tql_apply_cpp_core
+  # - lua_engine_cpp_core
+  # - join_tables_cpp_core
+  # - tql_to_sql_cpp_core
+  # - llm_anthropic_cpp_core
+uses_types: []
+framework: "imgui"
+entry_point: "main.cpp"
+dir_path: "cpp/apps/dag_engine_ui"
+repo_url: "https://gitea.organic-machine.com/dataforge/dag_engine_ui"
+---
+
+# dag_engine_ui
+
+Frontend ImGui para dag_engine. Lista, lanza e inspecciona DAGs con live updates via WS.
+
+## Build
+
+```bash
+cd cpp && cmake --build build --target dag_engine_ui -j
+```
+
+## Run
+
+```bash
+./cpp/build/dag_engine_ui
+```