Add, remove and move inverses restore the copy parent's remote-synced
flag with its touched groups. Swapping a child clears that flag during
delete; undo must restore the original parent state (F157).
Swap and add/remove regressions fail before the change. The focused swap
suite passes 49 tests and 191 assertions; common lint and format pass.
Benchmark ratios: mains 0.49/0.57/0.62; levels 0.53; chain 0.57; library
0.69; many shapes 0.62; switch 0.62, no regression flags.
AI-assisted-by: GPT-6
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