- Drag on empty space in Select tool draws a marquee rectangle;
elements intersecting it on release become selected
- Shift+click toggles elements in/out of the selection set
- Moving and deleting operate on all selected elements at once
- Middle mouse button drag now pans unconditionally in every tool mode
- Session selection state changed from Option<String> to Vec<String>
- New DragState::MarqueeSelecting variant with dashed rect rendering
- 29 tests passing, clippy clean
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