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/ui/ImGuiContext.cpp

@@ -98,6 +98,16 @@ bool ImGuiContext::Initialize(const std::string& windowTitle, int width, int hei
     // position overlay appears then); a plain drag just moves the window,
     // so casual rearranging can't accidentally re-dock panels.
     io.ConfigDockingWithShift = true;
+    // Windows move only via their title bar/tab, never by dragging empty
+    // content. Without this, a press-drag that a content widget doesn't
+    // capture falls through to ImGui's "drag body to move window" behaviour —
+    // which, in the node-editor canvas (whose background button doesn't
+    // reliably claim the drag in this nested-child embedding), meant dragging
+    // in the graph moved the whole Material Editor window. Title-bar dragging
+    // and Shift-to-dock are unaffected; this is also the desired behaviour for
+    // every other content area (viewports, timelines) where a content drag
+    // must never move the host window.
+    io.ConfigWindowsMoveFromTitleBarOnly = true;
 
     // Store imgui.ini in %APPDATA%\UsdLayerManager\ so it doesn't litter the CWD.
     if (const char* appData = getenv("APPDATA")) {