Function catch_up_keeps_gen 

fn catch_up_keeps_gen(open: &[u8], appended: bool) -> bool

Whether catching an epoch up over appended bytes keeps every counted line’s content, so it may keep its generation. Not when the counted prefix ends in an unterminated, counted line that the bytes continue: that line’s index would name different content (abc → abcdef), and if the continuation makes it blank (a split UTF-8 sequence completed into whitespace) the line vanishes and its index is reused for a later record (Codex on #3663). The count is still caught up — under a new generation.