feat: drag-reorder slots

pool.reorder() permutes slots (validated permutation) and keeps the
active selection on its slot; exposed via /grid_pool/reorder. The grid
thumbnails are drag handles; dropping on another cell reorders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gates/handlers.py

@@ -22,5 +22,9 @@ def handle_list(base, pool_id):
     return pool.read_manifest(base, pool_id)
 
 
+def handle_reorder(base, pool_id, order):
+    return pool.reorder(base, pool_id, order)
+
+
 def handle_set_mask(base, pool_id, index, mask_png_bytes):
     return pool.set_mask(base, pool_id, index, mask_png_bytes)  # Task 12