Exhibition site: repo, matrix build, GitHub Pages, hub

Part of #113. The exhibition site itself.

- Repo `holsee/cherry-exhibition`: content tree grown from the Juno Vale demo to exhibition grade (pages, 6-8 feature-stressing posts, full five-collection portfolio, search on).
- Workflow: matrix over themes; each leg `cherry build` with `theme: NAME`, `base_path: /t/NAME/`, `cherry check --strict`; one Pages artifact assembling all trees + the hub.
- Hub `index.html`: gallery cards (light/dark pairs from the harness, #115) linking into each theme's live tree.
- DNS: `themes.cherrybomb.dev` CNAME to GitHub Pages; CNAME file in the artifact.
- Switcher v1: a "Themes" nav entry back to the hub; the in-place floating pill follows once the island convention (#118) lands.

Start by reading the scope in #113 and the harness work in #115, then inspect the repository and the existing `cherry build` and `cherry check --strict` entry points. Define the matrix workflow, `index.html` hub, assembled Pages artifact, and its CNAME; confirm the switcher dependency on #118. Done means all theme trees, the hub links, DNS artifact, and checks work together.

Written by the indexing model from the issue text.