Function required_text 

fn required_text(
    pairs: &[(Value, Value)],
    key: &str,
    label: &str,
) -> Result<String, MoqTraceError>

Read a required text key, distinguishing “absent” from “present and not text”.

Both are malformed — there is no header to construct without the key — but they are different faults, and reporting a "perspective": 5 as a missing "perspective" sends whoever has to fix the file looking for something that is right in front of them. label names the key as a reader of the file would find it, which for a key inside "segment" is not its bare name.