Keep the roots' relative flip state when placing the source subtree.
Several small rotations must give the same child flags after a full sync;
recomputing one large turn could toggle both axes. Fix F139.
The direct regression failed nil/nil to true/true before the fix. Focused
geometry and oracle suites pass 20 tests, 88 assertions.
Benchmark ratios against Phase 1e: main 10/100/1000 0.57/0.64/0.60;
three levels 0.67; chain 0.56; library 0.60; many shapes 0.50;
variant switch 0.64. No regression flag.
AI-assisted-by: GPT-6
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