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/TimelinePanel.cpp

@@ -369,6 +369,11 @@ void TimelinePanel::SetCurrentFrame(double frame)
     }
 }
 
+void TimelinePanel::SetCurrentFrameExternal(double frame)
+{
+    SetCurrentFrame(frame);
+}
+
 void TimelinePanel::NotifyTimeChanged()
 {
     if (OnTimeChanged)