Go to History Step (and new Previous/Next History Step commands) loads the
step's settings and moves a per-photo cursor (`Photo.history_cursor`,
`Op::SetHistoryCursor`, catalog format v8; older catalogs open at the latest
step) instead of appending "History: …". Later steps stay listed, greyed in
the History panel, and can be returned to; a new edit made behind the end
drops them first. Each move is one undo step; undoing an edit restores the
dropped steps and the cursor. `history.list` returns `cursor`.
Also: spots.rs uses as_chunks (clippy chunks_exact_to_as_chunks).
Sources: own design (Lightroom Classic / Photoshop History panel behaviour, observed).
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
