When a file does not end with a newline, appending content extends the
last logical line's text and thus its visual height. The incremental
extend path in handle_file_appended only computed heights for newly
created logical lines, missing the old last line whose content changed.
Add VisualHeightIndex::replace_last_line_height() — an O(1) method that
rewrites the final prefix sum entry and total. Called before
extend_from_heights so the correct line is targeted.
Changes:
- progressive_reader.rs: add replace_last_line_height, pub with_params,
7 VHI unit tests
- app.rs: save old_reader_line_count before update, recompute last old
line height in extend path, 2 integration regression tests
d4679a7543