createSession property

Factory function to create a DragSession for this element.

When provided, ElementScope manages the session lifecycle:

• On drag start: calls createSession, then DragSession.start (locks canvas)
• On drag end: calls DragSession.end (unlocks canvas)
• On drag cancel: calls DragSession.cancel (unlocks canvas)

Elements manage their own business state (positions, sizes) internally. The session purely handles canvas lock coordination.

Example:

ElementScope(
  createSession: () => controller.createSession(),
  onDragStart: (_) => controller.startNodeDrag(nodeId),
  onDragUpdate: (details) => controller.moveNodeDrag(details.delta),
  onDragEnd: (_) => controller.endNodeDrag(),
  // ... other callbacks
)