Vendor node editor in-tree; add material presets; debug drag regression

Move the imgui-node-editor subset the build actually compiles from
third_party/imgui-node-editor to src/ui/NodeEditor (version-controlled,
MIT LICENSE included). FindImguiNodeEditor.cmake points at the new
location; CMakeLists excludes src/ui/NodeEditor from UI_SOURCES so it
isn't compiled twice.

Material editor presets: a Presets menu (disabled with no open material)
builds a UsdPreviewSurface + UsdUVTexture graph fed by an st primvar
reader, or a MaterialX standard_surface + image graph fed by a texcoord
node. Each is one idempotent, undoable command that re-normalizes layout.
Create Material becomes an icon button. New nodes route through
FindFreeCanvasSpot so a creation never lands on top of an existing node
(overlapping nodes fight over the editor hit test and become undraggable).

Shader-ball preview: pick a previewable output (terminal or 3/4-component
color-like) instead of always the first output, so scalar-only nodes keep
the whole-material preview rather than failing Storm codegen. Per-shape
camera frame-fit margins and auto-clip framing.

Fixes: DeletePrimCommand::Undo recreates missing destination ancestors
before SdfCopySpec (parent material may have been deleted after the
command ran). ConfigWindowsMoveFromTitleBarOnly stops a content-area drag
in the node canvas from moving the whole Material Editor window.

Temporary (marked for removal once the node-editor drag regression is
diagnosed): main.cpp mirrors LOG_INFO to %APPDATA%\UsdLayerManager\
debug.log; a g_AxNodeEditorDebugLog hook in the vendored editor plus a
[NodeGraph] event-trace block in RenderNodeGraphCanvas dump click/drag/
selection/position-save state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/MaterialEditorPanel.h

@@ -12,6 +12,7 @@
 #include <memory>
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 namespace UsdLayerManager {
 
@@ -82,6 +83,11 @@ private:
     void RenderMaterialBrowser();
     /// Creates a uniquely-named empty material under /Materials and opens it.
     void CreateNewMaterial();
+    /// Preset graphs for the open material (one undoable command each):
+    /// UsdPreviewSurface + UsdUVTextures fed by an st primvar reader, or a
+    /// MaterialX standard_surface + image nodes fed by a texcoord node.
+    void CreateUsdPresetGraph();
+    void CreateMaterialXPresetGraph();
     void RenderNodeGraphCanvas();
     void RenderPreviewPanel();
     void HandleCreateAndDelete();
@@ -89,6 +95,11 @@ private:
     /// creates the highlighted node at canvasPos.
     void RenderNodeSearchMenu(const ImVec2& canvasPos);
     void CreateShaderNode(const std::string& shaderId, const ImVec2& canvasPos);
+    /// Nudges a creation point right until the new node's estimated rectangle
+    /// clears every existing node's rendered rectangle. Overlapping nodes
+    /// fight over the editor's hit test: the buried one becomes unmovable and
+    /// its pins unhoverable. Must be called with the editor context current.
+    ImVec2 FindFreeCanvasSpot(ImVec2 desired, const std::string& shaderId) const;
     /// Binds an existing material to targetPath, undoably, restoring whatever
     /// direct binding (if any) targetPath had before.
     void BindMaterialToTarget(const pxr::SdfPath& materialPath, const pxr::SdfPath& targetPath);
@@ -153,6 +164,15 @@ private:
     /// is only valid inside the editor's Begin/End).
     std::string m_pendingCreateShaderId;
 
+    /// Debug event-trace state (see the logging block at the end of
+    /// RenderNodeGraphCanvas): whether the last LMB press landed on a node
+    /// rect, whether that press's drag-start has been logged yet, and the
+    /// editor NodeIds hit at press time (their live positions are logged on
+    /// every drag-move so a node that stops following the mouse is visible).
+    bool m_debugPressOnNode = false;
+    bool m_debugDragLogged  = false;
+    std::vector<uintptr_t> m_debugDragNodeIds;
+
     /// First node currently selected in the graph editor (empty = none);
     /// drives the properties section under the preview.
     pxr::SdfPath m_selectedNodePath;