Line-by-Line Diff

Implement a simple version of diff. Given two lists of lines, produce an edit script based on their longest common subsequence:

• diff_lines(a, b): a list of strings, one per line: " line" for unchanged lines, "- line" for removed lines and "+ line" for added lines. When there's a choice, output removals before additions at each change point.
• apply_diff(a, script): rebuild b from a and the script, checking that unchanged and removed lines match a (raise ValueError otherwise).
• diff_stats(script): {"added": n, "removed": n, "unchanged": n}.