v0.17.0 — Declarative metadata & layout

Foundation work for SPEC-078. The file-ref rune and the xref preview="drawer" extension both need a footer slot on the drawer rune and a flex-column drawer chrome so footer + header stay pinned while the body scrolls. Doing this once on the drawer rune itself unblocks both downstream items and lands a standalone-drawer enhancement at the same time (authors can now write {% drawer %}body --- footer{% /drawer %} directly).The single-block edge-to-edge body styling that was originally scoped here was pulled out after preview testing — the drawer-side :has(> .rf-codeblock:only-child, …) selector list couples the drawer to specific rune class names, which doesn't generalise (codegroup vs snippet vs chart vs sandbox each have different scroll-container needs; third-party runes can't extend the list). A cleaner shape (generic data-fill opt-in contract on the rune side, single generic :has(> [data-fill]:only-child) selector on the host side) is captured in SPEC-078's Future extensions and will land in a follow-up spec.