Function read_log_tail 

fn read_log_tail(path: &Path, max_lines: usize) -> String

Read the last max_lines lines of a file for embedding into an error message. Best-effort: an unreadable/missing file returns an empty string rather than an error — the caller already reports the path, we just enrich when we can. We read the whole file (soma.log is scribe-managed and stays small during boot), split, and take the tail — no seek-from-end acrobatics needed for the boot-time use case.