dropSlugClashes only compared against what was on disk, so a clip named like an
entity the model extracted wrote both pages and neither could be updated again.
In-batch winners: source over entity over concept. Against the brain, only
entities/concepts are skipped; the source's slug was already chosen free or its
own by resolveSourceSlug, so an existing bad pair no longer locks the source out.
Mutation-checked.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>