sliver_tree/reorder_render_port library

The narrow render-layer surface a drag-and-drop reorder session needs.

Keeping reorder code off the concrete RenderSliverTree avoids a variance cast, keeps sliver layout protocol out of gesture orchestration, and lets the controller be tested without a full widget tree. Same idiom as the codebase's other render-boundary contracts, AnimationReader and TreeRenderHost.

RenderSliverTree is the production implementation; tests exercise the reorder controller against scripted fakes.

Classes

ReorderRenderPort<TKey>

Render-layer contract consumed by TreeReorderController.