Function merge_into_template 

pub fn merge_into_template(
    template: &str,
    user_settings: &Map<String, Value>,
) -> String

Merge user settings into a JSONC template string.

For each user key:

• If the key exists as a commented-out line in the template (// "key": ...), that line is replaced with the uncommented user value.
• If the key is NOT in the template, it is appended before the closing }.

The result is always a valid JSONC file with the full template structure intact.