Library API notes

format_breaks returns Vec<Cow<'_, str>> rather than Vec<String>. Lines that are not thematic breaks are returned as Cow::Borrowed slices into the input, avoiding heap allocations for unchanged content. Synthesized thematic-break lines are also Cow::Borrowed, borrowing from a shared static buffer.

Callers that need owned String values must call .into_owned():

use mdtablefix::format_breaks;

let lines = vec!["some text".to_string(), "---".to_string()];
let owned: Vec<String> = format_breaks(&lines)
    .into_iter()
    .map(|c| c.into_owned())
    .collect();