Builder: visible drag-and-drop reordering

HTML5 drag gave no feedback (drop worked, but nothing moved on screen). Replace with a pointer-based sortable on the ⠿ grip:

• a neon chip follows the cursor,

• the dragged row dims (.dragging),

• rows shuffle LIVE as you move past their midpoints (array reorder + re-render each crossing), so you see the arrangement rearranging in real time.

▶ Play it: Neon Fiscal Year → Benthic Patience → Silken Reentry — a constellation across three families — the path is the mix https://stellate.app/?genre=citypop&seed=3775&path=1530.6221,586.10667,1376.19565&xdur=0.056

c3d4dad https://github.com/aboard-io/stellate/commit/c3d4dad787532d01e911d2da15597e20ca8536e0