#22 Tokyo Night diff coloring (dark+light)

Context: Owner review round — diffs are monochrome; want Tokyo Night colored diffs. Tasks:

1. Diff renderer (commits + PR patch.diff): line classes — hunk header @@ (cyan #7dcfff), +++/--- file lines (bold fg), added + (green #9ece6a on subtle bg #283b4d33? keep flat bg tint), removed - (red #f7768e tint), context (muted #565f89? keep normal fg), trailing-whitespace marker? skip. All inside escaped <pre>, spans with classes, colors as CSS vars (dark + light values per #19 palette — coordinate: define --diff-add etc. in both themes).
2. Keep truncation marker + binary note behavior. Word-diff? NO — line-level only (tiny).
3. Tests: fixture diff asserts class spans per line kind, escaping intact (<script> in added line escaped), light-theme vars defined. Success rubric:

• Green/red/cyan diffs in dark AND light; no unescaped HTML; truncation intact. Swarm: Owl. Branch m6/diff-colors. Coordinate CSS vars with #19 owner.