Machine-readable description of every registry field.
Used by the frontend <PolicyEditor> to render forms when a
Pydantic-driven OpenAPI schema isn’t sufficient (e.g., for the
JSONB sub-policy editors that need their sub-schema). Each entry
is the same shape as the x-ui block on the corresponding
OpenAPI property — section, label, control, bounds, role gates,
sub-schema. Stable across versions; the frontend reads it once
and caches client-side.