FindSlug returned the first hit, so a wing's own page of a name masked another
page of the same name (the retired dirs are searched before the wings, and wings
alphabetically). FindSlugs returns every match and callers refuse any that is
not their own. Found by the legacy-move planner's own test.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
With Config.Wing a source goes to wiki/<wing>/sources/ (wing/hall frontmatter);
concepts and entities stay in the shared wiki/concepts and wiki/entities, so an
entity cited from two wings is ONE page with both sources (the per-wing facts/
layout duplicated it). A source slug already used anywhere but its own wing's
sources/ is refused (slug is the graph identity). wiki.FindSlug added.
Mutation-checked: collision guard, shared inventory, per-wing source path.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>