Stage 17 round 6 (#3): drag-reorder rack tiles with a visual gap

Dragging a rack tile and dropping it back on the rack reorders it: the dragged tile is
lifted out (the drag ghost stands in) and the tiles at/after the pointer's drop slot slide
right to open a gap there, so the drop position is visible. On drop the rack and its stable
ids are permuted (reorderIndices, unit-tested). Reorder applies only with no pending tiles,
so it stays a clean permutation; dropping on a board cell still places as before. Server
persistence of the order follows (#4).

ui/src/lib/placement.test.ts

@@ -10,6 +10,7 @@ import {
   rackView,
   recallAt,
   recallIndex,
+  reorderIndices,
   reset,
   toSubmit,
 } from './placement';
@@ -122,3 +123,12 @@ describe('placementFromHint', () => {
     expect(p.pending.map((t) => t.letter)).toEqual(['A']);
   });
 });
+
+describe('reorderIndices', () => {
+  it('lifts an element and drops it at the given slot among the others', () => {
+    expect(reorderIndices(4, 0, 2)).toEqual([1, 2, 0, 3]);
+    expect(reorderIndices(4, 3, 0)).toEqual([3, 0, 1, 2]);
+    expect(reorderIndices(4, 1, 1)).toEqual([0, 1, 2, 3]); // back to identity
+    expect(reorderIndices(3, 0, 99)).toEqual([1, 2, 0]); // slot clamped to the end
+  });
+});