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/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;
 }