feat(shaders_lab): Output node + Functions palette with drag-drop

- DagKind::Output (new enum): terminal sink; compiler wires fragColor to its source_ids[0]
- dag_catalog: "output" node (1 input, red)
- dag_compile: skips Output in node_<i> emission; final fragColor resolves from Output's connection
- dag_node_editor: no more Add button; drops "DAG_NODE_TYPE" payloads at mouse canvas position; Output cannot be deleted; Output has no output pin
- dag_palette (new fn): Functions window with grouped, draggable node cards
- main.cpp: "Functions" window added; ensure_dag_default seeds plasma + connected Output

cpp/apps/shaders_lab/CMakeLists.txt

@@ -12,6 +12,7 @@ add_imgui_app(shaders_lab
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_uniforms.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_panel.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_node_editor.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/dag_palette.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/fps_overlay.cpp
 )
 target_include_directories(shaders_lab PRIVATE