fix(plugin): merge case-variant tags into a single archive page

[botbeech]

Summary

• _plugins/jekyll-paginate-tags.rb slugifies tag names via tag.downcase.strip.gsub(' ', '-').gsub(/[^\w-]/, ''), so distinct tag strings that share a slug (e.g. CTF + ctf, WebAssembly + webassembly) target the same /tag/<slug>/ destination.
• The old generator iterated site.tags.keys and emitted one page per tag name, so two tag names that slugified identically produced two pages at the same path. Jekyll logged Conflict: The following destination is shared by multiple files and the second page silently overwrote the first — dropping the posts of the losing tag from the archive.
• This PR groups tag names by slug before generation, unions + dedups their posts, and renders one page per slug.

Concrete example from the current post set

Three posts carry either ctf or CTF:

Post	Tag(s)
_posts/2012-12-31-29c3-ctf-misc-300-funchive.md	ctf
_posts/2014-07-01-binary-analysis-next-chapter-of-the-game.md	ctf
_posts/2018-10-14--flareon2018-chal5-webassembly-writeup-side-channel-attack.md	CTF

Before: /tag/ctf/ listed either two posts or one, depending on hash iteration order, and the build logged a conflict warning. After: /tag/ctf/ lists all three in reverse-chronological order.

Implementation notes

• Grouping happens on site.site_payload['site']['tags'] (the same Liquid-drop array the old code passed to Pager.new), so pagination, templating and paginator.posts keep their existing shape.
• Dedup by post['url'] protects against a post tagged with multiple case variants appearing twice on the merged page. Order is re-established via sort_by { |p| p['date'] }.reverse.
• Display name (page.tag in the layout) picks the first all-lowercase variant among the grouped names, falling back to the first seen. This matches the URL slug visually and keeps the rendered page headers readable.
• Empty slugs skipped: a tag whose characters are entirely stripped by the slug function would otherwise emit a page at /tag//. next if slug.empty? removes that edge case.
• _plugins/jekyll-paginate-authors.rb is not affected — authors are keyed by unique authors.yml usernames, not by free-form tag strings, so the collision pattern doesn't arise there.

Validation

Local bundle exec jekyll build (Jekyll 4.4.1) with the fix:

• Build completes in ~3s with no Conflict warnings.
• 56 _site/tag/<slug>/ directories produced.
• _site/tag/ctf/index.html contains links to all three CTF posts (verified with grep).
• _site/tag/bitcoin/page/{2,3}/ pagination still works for high-volume tags.
• All non-colliding tags (bitcoin, blockchain, cryptocurrency, etc.) render identically to before.

Relation to #87

This is the follow-up the Ruby/Jekyll upgrade PR (#87) flagged. Logically independent and applies cleanly on top of current master. Either order of review/merge works; the upgrade PR does not modify this plugin.