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/core/MaterialManager.cpp

@@ -185,6 +185,7 @@ ShaderGraphSnapshot MaterialManager::GetShaderGraph(const pxr::SdfPath& material
         pxr::SdrShaderNodeConstPtr sdrNode =
             pxr::SdrRegistry::GetInstance().GetShaderNodeByIdentifier(shaderId);
         if (sdrNode) {
+            node.category = DeriveCategory(sdrNode);
             for (const pxr::TfToken& inputName : sdrNode->GetShaderInputNames()) {
                 if (auto* prop = sdrNode->GetShaderInput(inputName))
                     node.inputs.push_back({inputName.GetString(), prop->GetTypeAsSdfType().GetSdfType()});

src/core/MaterialManager.h

@@ -39,6 +39,11 @@ struct ShaderPinInfo {
 struct ShaderGraphNode {
     pxr::SdfPath path;
     std::string  shaderId;
+    /// Coarse Hypershade-style bucket ("Material", "Texture", "Geometry",
+    /// "Utility", "Light") from the same classifier the create-node list uses;
+    /// empty when the shader id isn't in the Sdr registry. Drives which nodes
+    /// get an in-canvas thumbnail.
+    std::string  category;
     pxr::GfVec2f uiPosition{0.0f, 0.0f};
     /// False when no uiPosition custom data is authored (typical for networks
     /// referenced from .mtlx) — the editor auto-lays such nodes out instead.