[AbuMareBear]

Summary

• Extracts the sidebar footer into its own component, BridgetownStoa::SidebarFooter, rendered by bridgetown-stoa/layout in place of the inline markup. Markup is unchanged.
• Adds BridgetownStoa::Shadowable, a small mixin that makes a gem component look for its template in the host site's src/_components directory before falling back to the gem's copy. Bridgetown resolves a component's template next to the file that defines its Ruby class, which for a gem component is always inside the gem, so without this a host .serb would be silently ignored. (Serbea partials don't help either: they only resolve from the host's _partials dir.)
• Both Sidebar and SidebarFooter extend the mixin, so src/_components/bridgetown_stoa/sidebar.serb and .../sidebar_footer.serb now shadow the respective templates. Only the template is replaced; the Ruby class still comes from the gem.
• README gains an "Overriding parts of the layout" section with the shadow paths and a footer example; the whole-layout section now recommends component shadows for single-region changes.

Motivation

Guided-Rails/a11y-lint#115 copies Stoa's entire layout into the docs site just to add a "A Guided Rails project" line to the footer, and its header comment asks for a footer override point. After this ships, that copy can be replaced with a single sidebar_footer.serb.

Test plan

• script/cibuild — rubocop clean, 15 tests / 52 assertions pass
• New test/test_sidebar_footer.rb covers the default footer rendering inside <aside> and a host-site template shadowing it (writes and removes a fixture _components dir within the test)
• Verify live reload of a shadowed template under bin/dev (fast refresh matches on the resolved template path and reloads the component class, but I haven't watched it happen)

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