Compute the diff between two texts (a → b) — no git needed. Returns a standard unified patch (line-oriented in both modes, applies with git apply or patch), added/removed counts, and in word mode a token-level change list for inline highlighting (capped at 2000 entries). CRLF line endings and missing trailing newlines are preserved byte-exactly. Caps: 1 MB UTF-8 per input; pathologically divergent inputs return 422 diff_too_complex.