feat: add move_slide, duplicate_slide, and SlideShapes.add_copy_of()

[ska-sleepy]

Summary

Three slide/shape operations that have no native equivalent in the current API:

• Slides.move_slide(slide, position) — reorder a slide to any 0-based position in the sequence
• Slides.duplicate_slide(slide, position=None) — clone a slide; new slide lands after the source by default; related OPC parts (images, charts) are shared, not deep-copied
• SlideShapes.add_copy_of(source_shape) — append a deep copy of a shape from any slide onto this slide's shape tree; shapes with embedded parts share those parts with the source

Internals

• SlidePart.clone(source, package) — creates a new SlidePart from a deep copy of the source element, reproducing relationships in ascending rId order so XML rId references remain valid
• PresentationPart.duplicate_slide(source_slide_part) — delegates to SlidePart.clone and registers the new part

Limitations

• duplicate_slide: Notes slides attached to the source are not duplicated
• add_copy_of: For picture/chart shapes the underlying OPC parts are shared; editing the image or chart data affects both the original and the copy

Test plan

• tests/test_slide.py::DescribeSlides::it_can_move_a_slide
• tests/test_slide.py::DescribeSlides::it_does_nothing_when_move_slide_target_equals_current_position
• tests/test_slide.py::DescribeSlides::it_can_duplicate_a_slide
• tests/shapes/test_shapetree.py::DescribeSlideShapes::it_can_add_a_copy_of_a_shape_from_another_slide
• Full suite: 2688 passed (16 pre-existing errors unrelated to this PR)