Bulk removal inverses move siblings back in original order. Component
sync records each removal's current index; inverse sync falls back to
the original main container when the builder lacks it (F158).
A six-permutation deletion test and a main-swap sync test fail before the
fix. Focused suites pass 70 tests and 242 assertions; two inverse-sync
regressions also pass. Common lint and format pass. The controlled
benchmark has no flags: mains 0.51/0.59/0.58; levels 0.54; chain 0.58;
library 0.55; many shapes 0.51; switch 0.63. The first measurement under
concurrent stress load flagged main-100 at 1.17 and was rerun in isolation.
AI-assisted-by: GPT-6
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