v0.23.0 — Surface axes & the skeleton/skin extraction

The second slice of the SPEC-094 theme-platform epic. Where v0.22.0 tokenized type and built the gallery safety net, v0.23.0 attacks the differentiation surface: it extracts skeleton from skin and replaces the last cross-rune rune-name lists with engine-emitted, theme-overridable config.

Its centerpiece is SPEC-107 — the surface-axis decomposition. It amends SPEC-094 §8: the single surface enum (card/banner/inline/inset) splits into three orthogonal, theme-defaulted axes — an ordered elevation depth ladder (sunken/flush/flat/raised/floating, folding in today's shadow-only elevation), width carrying the bleed, and a page-section-family prominence scale (quiet/normal/prominent/display). The motivating outcome: the same recipe reads as a bordered card in prose and a full-bleed, large-title hero at the top of a page.