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/vendor/imgui-node-editor/examples/simple-example/CMakeLists.txt

@@ -0,0 +1,3 @@
+add_example_executable(simple-example
+    simple-example.cpp
+)

cpp/vendor/imgui-node-editor/examples/simple-example/simple-example.cpp

@@ -0,0 +1,63 @@
+# include <imgui.h>
+# include <imgui_node_editor.h>
+# include <application.h>
+
+namespace ed = ax::NodeEditor;
+
+struct Example:
+    public Application
+{
+    using Application::Application;
+
+    void OnStart() override
+    {
+        ed::Config config;
+        config.SettingsFile = "Simple.json";
+        m_Context = ed::CreateEditor(&config);
+    }
+
+    void OnStop() override
+    {
+        ed::DestroyEditor(m_Context);
+    }
+
+    void OnFrame(float deltaTime) override
+    {
+        auto& io = ImGui::GetIO();
+
+        ImGui::Text("FPS: %.2f (%.2gms)", io.Framerate, io.Framerate ? 1000.0f / io.Framerate : 0.0f);
+
+        ImGui::Separator();
+
+        ed::SetCurrentEditor(m_Context);
+        ed::Begin("My Editor", ImVec2(0.0, 0.0f));
+        int uniqueId = 1;
+        // Start drawing nodes.
+        ed::BeginNode(uniqueId++);
+            ImGui::Text("Node A");
+            ed::BeginPin(uniqueId++, ed::PinKind::Input);
+                ImGui::Text("-> In");
+            ed::EndPin();
+            ImGui::SameLine();
+            ed::BeginPin(uniqueId++, ed::PinKind::Output);
+                ImGui::Text("Out ->");
+            ed::EndPin();
+        ed::EndNode();
+        ed::End();
+        ed::SetCurrentEditor(nullptr);
+
+	    //ImGui::ShowMetricsWindow();
+    }
+
+    ed::EditorContext* m_Context = nullptr;
+};
+
+int Main(int argc, char** argv)
+{
+    Example exampe("Simple", argc, argv);
+
+    if (exampe.Create())
+        return exampe.Run();
+
+    return 0;
+}