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/core/commands/DeletePrimCommand.cpp

@@ -59,6 +59,14 @@ void DeletePrimCommand::Undo() {
     if (!rootLayer) return;
 
     try {
+        // The destination ancestors may be gone by now (e.g. the parent
+        // material was itself deleted after this command ran) — SdfCopySpec
+        // fails with "No spec ... when trying to set field 'primChildren'"
+        // when the parent spec is missing, so recreate the chain first.
+        pxr::SdfPath parentPath = m_primPath.GetParentPath();
+        if (!parentPath.IsAbsoluteRootPath() && !rootLayer->GetPrimAtPath(parentPath))
+            pxr::SdfCreatePrimInLayer(rootLayer, parentPath);
+
         if (!pxr::SdfCopySpec(m_savedLayer, m_primPath, rootLayer, m_primPath))
             LOG_ERROR("DeletePrimCommand::Undo: SdfCopySpec failed for " + m_primPath.GetString());
     } catch (const std::exception& e) {