OptionalenumRequired when type === 'enum'.
Dotted-name convention: <sourceMixinDomain>.<sub>...
OptionallifetimeDefault 'persistent'.
OptionalprivateDefault false. Private writes require actor === target.
OptionalvalidatorReturns true on success, an error message string on failure.
Schema for a single setting declared by a mixin's static
settingsfield. Seedocs/subsystems/shell-environment.md.