v0.22.0 — Theme platform foundations

The first slice of the SPEC-094 theme-platform epic: make the theme system ready to support themes beyond Lumina (magazine / editorial / business) — without yet building one. Two independent threads — tokenize + generate and gallery + visual-regression harness — plus the spike that sizes the v0.23.0 skeleton/skin extraction.

Deferred to later slices: the wholesale skeleton/skin extraction and data-surface (v0.23.0), and theme-owned fonts, new layouts, the theme scaffold, and an actual second theme (later).