v0.18.0

Added

• didactic-settings supports explicit list accumulation across composition
  layers: field+ appends and +field prepends in documents and dotted
  overrides. Extensions retain schema checks, tagged-union descent,
  interpolation, precedence order, schema defaults for unset lists, and
  per-element provenance; a later wholesale write still replaces the list.
  Selected fragments may now inherit sibling fragments through their own
  cycle-checked defaults list. (#85)