Function merge_layers 

pub fn merge_layers<'a, I>(layers: I) -> BTreeMap<ConfigPath, ConfigValue>
where
    I: IntoIterator<Item = &'a ConfigLayer>,

Merges many layers using shallow layer semantics.

Layers are applied in ascending priority order. Caller order is preserved for equal priorities, so later equal-priority layers replace earlier values. Map values are replaced as whole values; nested map merging is intentionally not part of this first version.