feat(shaders_lab): visual node editor (imgui-node-editor) + multi-source

- cpp/vendor/imgui-node-editor: vendorized thedmd/imgui-node-editor v0.9.4
- cpp/functions/gfx/dag_node_editor: new visual pipeline editor replacing dag_panel
- DagStep: source_ids[4] + editor_pos + editor_uid (multi-input support)
- DagNodeDef: num_inputs explicit; circle reclassified as Op
- dag_compile: N inputs per node, topological ordering preserved
- main: use node editor; destroy on shutdown
- patch imgui_extra_math.inl: guard operator*(float, ImVec2) for imgui >= 18955

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cpp/apps/shaders_lab/CMakeLists.txt

@@ -11,11 +11,13 @@ add_imgui_app(shaders_lab
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_compile.cpp
     ${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/core/fps_overlay.cpp
 )
 target_include_directories(shaders_lab PRIVATE
     ${CMAKE_CURRENT_SOURCE_DIR}
 )
+target_link_libraries(shaders_lab PRIVATE imgui_node_editor)
 
 if(WIN32)
     # GUI app: sin consola al lanzar (subsystem:windows / -mwindows)