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)

cpp/apps/shaders_lab/main.cpp

@@ -9,6 +9,7 @@
 #include "gfx/dag_compile.h"
 #include "gfx/dag_uniforms.h"
 #include "gfx/dag_panel.h"
+#include "gfx/dag_node_editor.h"
 #include "core/fps_overlay.h"
 #include "seed_shaders.h"
 
@@ -142,7 +143,7 @@ static void render() {
 
     // --- DAG Pipeline window ---
     if (ImGui::Begin("DAG Pipeline")) {
-        if (fn::gfx::dag_panel(g_pipeline)) {
+        if (fn::gfx::dag_node_editor(g_pipeline)) {
             g_dag_dirty = true;
         }
         draw_err(g_dag_err, g_dag_err_line);
@@ -205,5 +206,6 @@ int main() {
     int rc = fn::run_app(cfg, render);
     fn::gfx::canvas_destroy(g_canvas_code);
     fn::gfx::canvas_destroy(g_canvas_dag);
+    fn::gfx::dag_node_editor_destroy();
     return rc;
 }