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/functions/gfx/dag_node_editor.h

@@ -0,0 +1,17 @@
+#pragma once
+#include <vector>
+#include "gfx/dag_types.h"
+
+namespace fn::gfx {
+
+// Renders the visual node editor (imgui-node-editor) inside the current ImGui::Begin.
+// Modifies pipeline in-place: add/remove nodes, manage edges (source_ids).
+// Returns true if TOPOLOGY changed (nodes or edges added/removed).
+// Does not return true for position moves or slider changes.
+// Call once per frame. Maintains its own editor context (static singleton).
+bool dag_node_editor(std::vector<DagStep>& pipeline);
+
+// Release editor resources. Call on shutdown.
+void dag_node_editor_destroy();
+
+} // namespace fn::gfx