Compare each side with its preceding source row within the same excerpt,
not the adjacent aligned pair: alignment holes must not restart labels.
Retain sticky context independently on each side, including leading holes.
Regression: merged Python insert/delete hunks emitted five labels instead
of one. Verify deduplication, folded excerpt restart, and sticky-gap context.
Verified: cargo test (950 passed), cargo build, and real PTY smoke covering
split/unified, wrap, sticky gaps, and a short horizontal pane source caret.
Valdemar Lorenzen
39fd3a669b