Hook up menu-bar actions that lacked a mobile affordance, and implement z-order.
- Stage/object long-press menu (stage.rs): add Paste (on clip, geometry, and
empty stage) and Send to Back / Bring to Front on the selection.
- Timeline long-press menu (timeline.rs): the timeline had no mobile context
menu — add one via manual long-press detection. Clip actions (Split,
Duplicate, Cut, Copy, Paste, Delete) on a clip; animation actions (New/Blank
keyframe, Add keyframe at playhead, Duplicate keyframe, Delete frame, Add
motion/shape tween) on an empty lane. Gate the desktop right-click menu to
!is_mobile so mobile shows only the long-press menu.
- Implement Send to Back / Bring to Front (were // TODO no-ops): new undoable
ReorderClipInstancesAction in lightningbeam-core reorders the selected
instances within their layer's clip_instances Vec (stacking order; last =
on top; geometry stays flattened underneath). Wired via handle_menu_action.
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