feat(shell): config-driven topbar links (GitHub/social) with shipped brand marks

[mhenrixon]

Summary

Adds c.topbar_links — a per-site list of repo/social links rendered as icon-only ghost buttons in the topbar, next to the theme switcher. The headline case is a GitHub repo link with a real GitHub logo (this PR dogfoods it: the docs site now links to mhenrixon/docs-kit).

c.topbar_links = [
  { href: "https://github.com/you/repo", label: "GitHub",  icon: :github },
  { href: "https://discord.gg/invite",   label: "Discord", icon: :discord },
  { href: "/changelog",                  label: "Changelog", icon: "history" }, # a lucide icon
]

Why ship our own brand marks

lucide (the kit's synced icon set) ships no brand logos — it dropped its brand icons, and rails_icons' brand library (simple_icons) is gone from the current icons gem. Syncing tabler's brand-* glyphs would clone ~5,900 SVGs into every site. So the kit ships its own small, curated set of developer/social marks as inline SVG (DocsUI::BrandMark), each an official Simple-Icons 24×24 single-path glyph, verified byte-exact against simpleicons.org:

:github · :gitlab · :discord · :x · :rubygems · :bluesky · :mastodon · :slack · :whatsapp · :telegram · :linkedin · :youtube · :reddit · :stackoverflow

An icon: that isn't a shipped brand is treated as a lucide name and delegated to DocsUI::Icon; omitting icon: renders the label as a text button. External links open in a new tab with rel="noopener"; site-relative links open in place. Marks use fill: currentColor, so they recolor with the active daisyUI theme.

Backwards compatible: c.topbar_links defaults to [] — a site that sets nothing has a byte-identical topbar.

What changed

Layer	Change
Config	DocsKit::Configuration#topbar_links (default [], normalizes Hashes → value objects)
Value object	DocsKit::TopbarLink (href/label/icon, #external?, .from)
Component	DocsUI::BrandMark (inline brand SVG + lucide fallthrough)
Component	DocsUI::TopbarLinks (renders the links); DocsUI::Shell composes it before the theme switcher
Install path	commented c.topbar_links example in the initializer template (so docs-kit new sites get it)
Docs	README section + component table; dogfooded in the docs site

No new CSS class is emitted — btn/btn-square/btn-ghost/size-5/flex-none are already-scanned literals — so no @source change is needed.

Test plan

• config — default [], Hash normalization, declaration order, nil → [], accepts TopbarLink objects
• TopbarLink — .from (Hash/object, string keys, icon coercion), #external?
• BrandMark — .brand?; all 14 brands present + valid 24×24 single-path SVGs; brand render (viewBox / currentColor / role=img / <title>); String key; lucide fallthrough
• TopbarLinks — brand mark, aria-label, external target/rel, relative link, icon-less text button, order, empty → nothing
• Shell — links render before the switcher; unchanged when unset
• Generator — initializer documents the commented knob

Verification

• bundle exec rspec — 654 examples, 0 failures, 96% line coverage
• bundle exec rubocop — 121 files, no offenses
• docs site CSS rebuilt (bin/build-css); all emitted classes present

https://claude.ai/code/session_01FPQb6z3YwcKRMbvoJhdxnX