Rework 3-pane divider behavior and animate every layout change:
- All transitions now ease over ~100ms via a LayoutAnim between two full stack
configs (resize, slide, collapse, fullscreen) instead of jumping; edge
transitions continue from the drag's progress.
- 3-pane dividers are linked along a single path parameter through the three
presets ([.5,.25,.25] / even / [.25,.25,.5]) so both dividers move together
and reach their snaps simultaneously. Over-grow past the extreme preset is
anchored to the path endpoint so the non-dragged divider stays continuous.
- Growing a pane past ~66% collapses the window to 2 panes (dropping the
opposite-end pane); the surviving divider snaps to the nearest 2-pane preset
(1/4, 1/2, 3/4) based on the drop position. Dragging to the slide extreme
still slides the window.
- 2-pane snaps to 1/4, 1/2, 3/4.
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