Recalculate cell positions in the previous grids after moving children.
The destination update alone left cell references to shapes that moved.
Validation: reset and move suites, 21 tests / 92 assertions. The grid
move regression failed before the fix and checks exact undo.
Benchmark ratios against Phase 1e: main 10/100/1000 copies
0.54/0.56/0.60; three levels 0.52; chain 0.52; library 0.55;
many shapes 0.58; variant switch 0.61. No regression flag.
AI-assisted-by: GPT-6
e4e02a4753