A theme override aimed at a composed rune is reported and dropped
ADR-042 makes a composed rune's config its definition's: a theme cannot know which definition it faces, so an override of a composed rune's blocks or metaFields is a guess that silently resolves to nothing. Nothing enforces this yet. mergeThemeConfig and mergeRuneConfig (packages/transform/src/merge.ts) guard the identity fields on every rune, and nothing else, so a theme's runes: entry keyed to a composed rune's type name applies.
v0.44.0 adds the first composed runes a theme could try to override (hero, cta, testimonial), so the guard lands with them.