During a Selection-tool drag the bounding box and its handles stayed on
screen while the art moved, so the content looked as if it slid inside
a box of its own. The box now hides once a drag moves, scales or
rotates the selection, as in Illustrator, and comes back on release. A
press that hasn't moved, and a marquee, keep it. Tools gain
`transforming()` (Session::tool_transforming) for this.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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