Add animation curve editor with Bezier F-curve round-trip

Maya-style Graph Editor panel (View > Curve Editor) that fits Catmull-Rom
Bezier curves through USD time samples, allows interactive editing with
tangent handles, and bakes the result back to dense linear samples on Apply.

Key behaviours:
- Left panel lists all animated attributes of the selected prim, decomposed
  into per-component channels (translate [X/Y/Z], etc.) with colour swatches
  and visibility toggles
- ImDrawList canvas: smooth Bezier polylines, always-visible tangent handle
  lines/circles, diamond keyframe markers
- Pan (MMB/Alt+drag), zoom (scroll / Shift+scroll), Frame All (F)
- LMB drag moves keyframes; tangent handle drag reshapes curve with mirrored
  or broken handles; box-select for multi-selection
- Double-click canvas adds a keyframe; Delete removes selected keyframes
- RMB context menu: Delete, Flatten, Break/Unify Tangents, Auto Tangents
- Simplify toggle (Ramer-Douglas-Peucker) reduces baked sample count
- Time cursor draggable to scrub the timeline
- UsdNotice::ObjectsChanged listener re-fits clean channels whenever the
  stage changes externally (Property Panel edits, Auto-Key, undo/redo),
  while preserving channels with unsaved Bezier edits
- Bake is a single undoable AttributeSetCommand (Ctrl+Z restores original
  sparse samples); Revert discards in-editor edits without touching USD

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/Application.cpp

@@ -43,6 +43,7 @@ Application::Application()
     , m_showViewport(true)
     , m_showPropertyPanel(true)
     , m_showTimeline(true)
+    , m_showCurveEditor(false)
     , m_running(false) {
 }
 
@@ -78,11 +79,19 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
     m_propertyPanel->SetPropertyManager(m_propertyManager.get());
     m_propertyPanel->SetCommandHistory(&m_commandHistory);
 
+    m_curveEditorPanel = std::make_unique<CurveEditorPanel>();
+    m_curveEditorPanel->SetCommandHistory(&m_commandHistory);
+
     m_timelinePanel = std::make_unique<TimelinePanel>();
     m_timelinePanel->OnTimeChanged = [this](pxr::UsdTimeCode displayTime,
                                             pxr::UsdTimeCode editTime) {
         m_viewportPanel->SetTimeCodes(displayTime, editTime);
         m_propertyPanel->SetTimeCodes(displayTime, editTime);
+        m_curveEditorPanel->SetTimeCodes(displayTime, editTime);
+    };
+
+    m_curveEditorPanel->OnScrub = [this](double frame) {
+        m_timelinePanel->SetCurrentFrameExternal(frame);
     };
     m_timelinePanel->OnBrowseFolder = [this]() -> std::string {
         HWND hwnd = m_imguiContext ? m_imguiContext->GetWindowHandle() : nullptr;
@@ -101,6 +110,7 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
         [this](const std::string& path) {
             m_viewportPanel->SetSelectedPrimPath(path);
             m_propertyPanel->SetSelectedPrimPath(path);
+            m_curveEditorPanel->SetSelectedPrimPath(path);
         });
 
     m_sceneHierarchyPanel->SetOnStageMetadataChanged(
@@ -108,16 +118,18 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
             RefreshManagers();
         });
 
-    // Single click in viewport → sync hierarchy + property panel
+    // Single click in viewport → sync hierarchy + property panel + curve editor
     m_viewportPanel->OnPrimPicked = [this](const std::string& path) {
         m_sceneHierarchyPanel->SetSelectedPath(path);
         m_propertyPanel->SetSelectedPrimPath(path);
+        m_curveEditorPanel->SetSelectedPrimPath(path);
     };
 
-    // Rect drag in viewport → sync hierarchy + property panel (primary path)
+    // Rect drag in viewport → sync hierarchy + property panel + curve editor (primary)
     m_viewportPanel->OnPrimsPickedRect = [this](const std::vector<std::string>& paths) {
         m_sceneHierarchyPanel->SetSelectedPaths(paths);
         m_propertyPanel->SetSelectedPrimPath(paths.empty() ? "" : paths.front());
+        m_curveEditorPanel->SetSelectedPrimPath(paths.empty() ? "" : paths.front());
     };
 
     if (!m_stageManager->CreateInMemoryStage()) {
@@ -178,6 +190,7 @@ void Application::RefreshManagers() {
         m_viewportPanel->FrameScene();
         m_propertyPanel->SetStage(stage);
         m_timelinePanel->SetStage(stage);
+        m_curveEditorPanel->SetStage(stage);
     } else {
         m_layerManager->SetStage(nullptr);
         m_propertyManager->SetStage(nullptr);
@@ -185,6 +198,7 @@ void Application::RefreshManagers() {
         m_viewportPanel->SetStage(nullptr);
         m_propertyPanel->SetStage(nullptr);
         m_timelinePanel->SetStage(nullptr);
+        m_curveEditorPanel->SetStage(nullptr);
     }
 }
 
@@ -255,6 +269,13 @@ void Application::RenderUI() {
         ImGui::End();
     }
 
+    if (m_showCurveEditor) {
+        ImGui::SetNextWindowSize({960, 320}, ImGuiCond_FirstUseEver);
+        ImGui::Begin("Curve Editor", &m_showCurveEditor, ImGuiWindowFlags_NoCollapse);
+        m_curveEditorPanel->Render();
+        ImGui::End();
+    }
+
     // Capture happens after the timeline so that the "Start" button click in
     // this frame is detected, and after the viewport has rendered the scene.
     if (m_timelinePanel->IsPlayblasting()) {
@@ -395,6 +416,7 @@ void Application::RenderMenuBar() {
             ImGui::MenuItem("Viewport",         nullptr, &m_showViewport);
             ImGui::MenuItem("Property Panel",   nullptr, &m_showPropertyPanel);
             ImGui::MenuItem("Timeline",         nullptr, &m_showTimeline);
+            ImGui::MenuItem("Curve Editor",     nullptr, &m_showCurveEditor);
             ImGui::Separator();
             ImGui::MenuItem("Stage Info",       nullptr, &m_showStageInfo);
             ImGui::MenuItem("Demo Window",      nullptr, &m_showDemoWindow);