S23.11: Brand token refresh, scroll logo, and cososo px type scale #613

Parent epic: #246

Tighten the docs-site branding toward cososo.co.uk per the design handoff, in brand.css + one small JS only, keeping our header/footer/logo/favicon/nav layout. Tokens (#626262 body, ~1.73 line-height, #3e2199 accent), scroll-shrink logo (1.0->0.755 past 48px), and a fixed-px content type scale (body 16, h1 29/h2 26/h3 23/h4 20/h5 18/h6 16) so content stops scaling with Material's viewport-scaled rem root. Rejected the handoff's drop-in mkdocs.yml, favicon swap, and extra.css. Follow-up: device/pixel-density/mobile scale review once live.