Ask before reopening a bundle that carries no cuts

A bundle without a source block cannot be round-tripped, but its PDF can
still be cut from scratch. Offer that instead of refusing: confirm, run
detection, and line the markers up by position when the slice counts
match exactly.

docs/guide.md

@@ -117,6 +117,15 @@ It refuses to overwrite a PDF or project file that is already there, since the
 obvious place to unpack is exactly where someone's unfinished work lives — pass
 `--pdf elsewhere.pdf` or `--force` if you mean it.
 
+A bundle from a producer that does not record its cuts can still be opened, but
+it is a fresh session rather than a round trip: it asks first, then cuts the PDF
+from scratch with detection. The title block always comes back. Markers and
+re-engraved systems land only if detection happens to find exactly as many
+slices as the bundle has — the slices are in reading order on both sides, so
+they can be lined up, but one system found or missed would shift every marker
+onto the wrong slice, so in that case they are left off entirely and it says so.
+`--detect` answers the question in advance, for scripts.
+
 ## Re-engraving a slice (optional, needs LilyPond)
 
 When a system is beyond rescue — a bad scan, a wrong transposition, a passage