Function default_basename 

pub fn default_basename(started_at_unix_secs: u64) -> String

Compose the extension-less base filename for a session that started at started_at (Unix epoch seconds). Format Screen-YYYY-MM-DD-HHMMSS — sortable, unambiguous, file-system- safe on every OS. M-SAVE.1’s Save panel appends the extension matching the chosen export format.