v0.20.1 — Surface model: docs & showcase

The composability investigation surfaced ~25 meaningful rune combinations — far more than fit in the authoring guide. Create a dedicated "Compositions" docs category, page-per-pattern: each page shows the Markdown you write, the rendered result (via preview), and a one-liner on which zone/mechanism makes it work. Start with Family A (a visual rune in a container's media zone), the set being made first-class this milestone.

The v0.20.0 surface model shipped with per-rune reference docs (card cover, recipe cover, bg, surfaces.md) but no single page that shows the vocabulary as a system. Build a dedicated showcase gallery that demonstrates the whole surface model on one page — the "wow" surface a visitor lands on to grasp what the model can do — organised by the model's own axes.