Function update_key16_toml_text 

pub fn update_key16_toml_text(text: &str, key: [u8; 16]) -> String

Text-only helper used by the writer and regression tests.

key is preferred over the legacy key_hex spelling because the parser also gives key precedence. Existing multiline key = [...] assignments are collapsed to one canonical line when their closing bracket can be identified without crossing another assignment/table header.