Two pages that slugify to the same string (e.g. both named "About")
previously both published to about.html, silently overwriting each
other on publish. Add a pure uniqueSlug(base, existingSlugs) helper
that appends -2, -3, ... on collision, and apply it in addPage,
renamePage, and replaceAllPages. The landing page's slug stays locked
to 'index' regardless of collisions, matching existing behavior.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
