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/docs/TODO.md

@@ -0,0 +1,14 @@
+In random order:
+* Documentation: Make one
+
+Done:
+* ~~ImGui: Factor out changes to ImGui to use vanilla version.~~
+* ~~Editor: Fix variable naming (mainly add `m_` prefix)~~
+* ~~Editor: Split NodeEditorImpl.cpp to multiple files, file has grown too big.~~
+* ~~Editor: Factor out use of `picojson.h`~~
+* ~~Editor: Move use of `<optional>` to optional code extensions~~
+
+
+
+
+#57 - join `ax::NodeEditor::EditorContext` with `struct EditorContext` and remove `reinterpret_cast<>`