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/bundle-format.md

@@ -228,6 +228,14 @@ The archived document is the source of truth for reopening: the slice images are
 output, and a consumer that reopens a bundle re-renders them rather than
 importing them.
 
+`source` is optional, so a bundle without one is still valid — it is simply not
+reopenable, and a consumer should say so rather than pretend otherwise. What it
+can still recover from such a bundle is the title block, and, if it cuts the
+document again and happens to find exactly as many slices, the markers: the
+slices array is in reading order, so it lines up with any other list in reading
+order. One slice more or fewer and it does not, which is why that is a fallback
+and not the design.
+
 ## Engraving
 
 Most slices are photographs of print: an image and nothing more. A slice that