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>

cmake/modules/FindImguiNodeEditor.cmake

@@ -1,9 +1,13 @@
 # FindImguiNodeEditor.cmake - Find or configure thedmd/imgui-node-editor
 #
-# This module looks for imgui-node-editor in the third_party/imgui-node-editor
-# directory (vendored, docking branch) and compiles its sources directly
-# against this project's own Imgui::Imgui target so it shares the exact same
-# ImGui headers/defines (no second copy of ImGui is pulled in).
+# This module looks for imgui-node-editor at src/ui/NodeEditor (an in-tree,
+# version-controlled subset of the upstream library — MIT licensed, see
+# src/ui/NodeEditor/LICENSE — containing only the files this project's build
+# actually compiles; the full upstream drop, including the standalone example
+# app used to reproduce library-level bugs against a pure reference build,
+# stays vendored separately at third_party/imgui-node-editor) and compiles its
+# sources directly against this project's own Imgui::Imgui target so it shares
+# the exact same ImGui headers/defines (no second copy of ImGui is pulled in).
 #
 # Inputs:
 #   IMGUI_NODE_EDITOR_DIR - Path to imgui-node-editor/NodeEditor root
@@ -13,12 +17,12 @@
 #   ImguiNodeEditor::ImguiNodeEditor  (imported STATIC target)
 
 if(NOT IMGUI_NODE_EDITOR_DIR)
-    set(IMGUI_NODE_EDITOR_DIR "${CMAKE_SOURCE_DIR}/third_party/imgui-node-editor/NodeEditor")
+    set(IMGUI_NODE_EDITOR_DIR "${CMAKE_SOURCE_DIR}/src/ui/NodeEditor")
 endif()
 
 if(NOT IMGUI_NODE_EDITOR_BLUEPRINT_UTILITIES_DIR)
     set(IMGUI_NODE_EDITOR_BLUEPRINT_UTILITIES_DIR
-        "${CMAKE_SOURCE_DIR}/third_party/imgui-node-editor/Examples/Common/BlueprintUtilities")
+        "${CMAKE_SOURCE_DIR}/src/ui/NodeEditor/BlueprintWidgets")
 endif()
 
 find_path(ImguiNodeEditor_INCLUDE_DIR
@@ -48,7 +52,7 @@ endforeach()
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(ImguiNodeEditor
     REQUIRED_VARS ImguiNodeEditor_INCLUDE_DIR
-    FAIL_MESSAGE "imgui-node-editor not found — expected it vendored at third_party/imgui-node-editor/NodeEditor"
+    FAIL_MESSAGE "imgui-node-editor not found — expected it at src/ui/NodeEditor"
 )
 
 if(ImguiNodeEditor_FOUND AND ImguiNodeEditor_SOURCES_MISSING)