Add in-canvas node thumbnails to material editor graph

Texture nodes show their decoded source image (background thread);
material nodes show a shader-ball render, amortized one-per-frame.
Also classifies shader nodes by Hypershade-style category and adds
an Arnold preset graph alongside the existing USD/MaterialX ones.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ui/MaterialEditorPanel.h

@@ -4,6 +4,7 @@
 #include "../core/MaterialManager.h"
 #include "IconManager.h"
 #include "MaterialPreviewRenderer.h"
+#include "NodeThumbnailCache.h"
 #include <pxr/usd/usd/stage.h>
 #include <pxr/usd/sdf/path.h>
 #include <pxr/usd/sdf/valueTypeName.h>
@@ -88,6 +89,7 @@ private:
     /// MaterialX standard_surface + image nodes fed by a texcoord node.
     void CreateUsdPresetGraph();
     void CreateMaterialXPresetGraph();
+    void CreateArnoldPresetGraph();
     void RenderNodeGraphCanvas();
     void RenderPreviewPanel();
     void HandleCreateAndDelete();
@@ -113,6 +115,9 @@ private:
     void CreateConnection(const PinInfo& destInput, const PinInfo& sourceOutput);
     void DeleteNode(const pxr::SdfPath& nodePath);
     void DisconnectAttr(const pxr::SdfPath& destNode, const std::string& destInput);
+    /// Resolved on-disk path of a texture node's image (its Asset-typed `file`
+    /// input), or empty if unauthored/unresolvable. Feeds the thumbnail cache.
+    std::string ResolveTextureFilePath(const ShaderGraphNode& node) const;
     void SyncFromUsd();
     void PersistNodePosition(ax::NodeEditor::NodeId nodeId);
     bool IsPinLinked(const pxr::SdfPath& nodePath, const std::string& pinName, bool isOutput) const;
@@ -146,6 +151,8 @@ private:
     std::unordered_map<uintptr_t, LinkInfo> m_linkIdToInfo;
 
     MaterialPreviewRenderer m_preview;
+    /// Per-node in-canvas thumbnails (texture images + material shader balls).
+    NodeThumbnailCache m_thumbnails;
 
     ImVec2 m_pendingCreateNodePos{0.0f, 0.0f};