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/main.cpp

@@ -5,6 +5,7 @@
 #include <exception>
 #include <Windows.h>
 #include <string>
+#include <filesystem>
 
 static void SetUsdPluginPath() {
     char exePath[MAX_PATH];
@@ -98,7 +99,16 @@ static void SetUsdPluginPath() {
 int main(int /*argc*/, char* /*argv*/[]) {
     try {
         UsdLayerManager::Logger::Instance().SetLogLevel(UsdLayerManager::LogLevel::Info);
-        
+        // Temporary: capture LOG_INFO to a file so the [NodeGraph]/[ed] debug
+        // traces are readable after the fact (this is a WIN32-subsystem app,
+        // so stdout isn't visible when launched normally). Remove once the
+        // node-editor drag regression is diagnosed.
+        if (const char* appData = getenv("APPDATA")) {
+            std::filesystem::path logDir = std::filesystem::path(appData) / "UsdLayerManager";
+            std::filesystem::create_directories(logDir);
+            UsdLayerManager::Logger::Instance().SetLogFile((logDir / "debug.log").string());
+        }
+
         LOG_INFO("=== USD Layer Manager Starting ===");
 
         SetUsdPluginPath();