Feat: self-host the type system and use the wordmark in the header

[SandObserver]

Summary

• Replace the Google Fonts stylesheet and its two preconnects with a self-hosted Latin subset of the Inter variable font at public/fonts/inter-latin.woff2, 55 KB covering weights 100 to 900 and the optical size axis, declared in a new src/styles/fonts.css and preloaded on the landing page and every documentation page.
• Point --font-ui and --sy-font at Inter with the system stack as the fallback, and collapse --font-display and --sy-font-display onto them so headings and body text share one family.
• Replace the landing page header and footer brand, previously the mark plus a styled <span>, with stackyard-wordmark-dark.svg at its intrinsic ratio.
• Set the Starlight logo to the light and dark wordmarks with replacesTitle, and drop the now inert .site-title font rule from docs.css.
• Ship Inter's OFL text beside the font file.
• Add woff2 to the asset extensions check-dist.mjs verifies, so a referenced font that never ships fails the gate.

Testing
npm run build and npm run check both pass. Verified in the build output that no page references Google's font hosts, that the font ships and is preloaded, and that the documentation header emits both wordmarks with Starlight's light and dark swap classes.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.