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>

CMakeLists.txt

@@ -173,6 +173,11 @@ endif()
 # Collect source files
 file(GLOB_RECURSE CORE_SOURCES "src/core/*.cpp")
 file(GLOB_RECURSE UI_SOURCES "src/ui/*.cpp")
+# src/ui/NodeEditor is the in-tree vendored imgui-node-editor subset — already
+# compiled into the imgui_node_editor_impl target by FindImguiNodeEditor.cmake
+# (with its own required include dirs/defines); excluded here so it isn't
+# compiled a second time, incorrectly, as part of this target's own sources.
+list(FILTER UI_SOURCES EXCLUDE REGEX "/src/ui/NodeEditor/")
 file(GLOB_RECURSE UTILS_SOURCES "src/utils/*.cpp")
 set(MAIN_SOURCE "src/main.cpp")