Scroll Deck Variant 2 + First-Class Navigation Chrome (Header Toggle, Variant Registry, Reusable Nav Primitives)

Promoted full-deck scroll variants to a first-class deliverable alongside the slide-by-slide variant chooser. Shipped a second 17-slide composition (`/thesis/version-2`) with substantially different layout choices per slide, then built the navigation chrome that lets the work be discovered: a CTA on the index page, a header that surfaces both surfaces (TOC + Scroll), an inline variant toggle that appears only when a scroll deck is loaded, and a registry-driven `DeckNav` component reused by both surfaces.