History diff: keep a stepped-back object in place; number hunks right

Rewind put an object's old form at the end of its list, so a history
entry's diff showed the changed object moving (turning hairpin off for
one forward also moved its forward and DNAT lines). It now sorts after
stepping back. go-udiff v0.4.1 numbers the new side of hunks after one
with a moved line too early, so the chain after @@ could be the one
before; hunkStarts numbers them again. History diffs name their sides
"before N" and "after N".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/engine.go

@@ -335,6 +335,8 @@ func Rewind(st *model.State, h *model.HistoryEntry) error {
 			return err
 		}
 	}
+	// put appends what it brings back; its position puts it in place.
+	st.Sort()
 	return nil
 }