v0.43.0 — Section is the composition base

A composition that wants a titled region with media places {% section %}, not {% card %}. section gains a body role, a header grammar that no longer reads prose as a header, the media split, cover and a media kind. The composed storytelling runes move onto it, and two new primitives arrive: {% actions %} for rows of buttons and a block-level {% link %}. card renders exactly as it does today (ADR-041, ADR-046, SPEC-160).