v0.26.0 — Editorial reading & shared layout modes

Continues the SPEC-094 theme-platform arc by settling the two capability gaps it deferred: the reading register for body text (the last reserved §8 candidate) and the promotion of carousel from a gallery-coupled behavior to a canonical layout mode any rune can opt into. Where v0.23.0 governed a rune's container (elevation/prominence/width), this milestone reaches inside it — how the running text reads, and how homogeneous items lay out.