Module settings_update 

The two types that sit beside Settings: a partial update, and what validation refuses.

A #[path] sibling of settings.rs rather than a section of it. The struct and the enum are each a flat list that grows by one line per setting, and settings.rs is at its size baseline with Settings itself still to grow; splitting the lists out is the only part of that file that can move without taking the Settings struct the surface checks parse out of the file they parse.

Structs

SettingsUpdate

Partial settings update.

Enums

SettingsError

Settings validation error.