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.

noteman_slicer/cli.py

@@ -102,20 +102,50 @@ def _export(args: argparse.Namespace) -> int:
     return 0
 
 
+def _confirm(question: str) -> bool:
+    """Ask before doing something the caller did not ask for. No tty, no."""
+    if not sys.stdin.isatty():
+        print(f"{question} (not a terminal — pass --detect to say yes)")
+        return False
+    return input(f"{question} [y/N] ").strip().lower() in ("y", "yes")
+
+
 def _open(args: argparse.Namespace) -> int:
     from . import bundle
     from .editor import launch
 
+    zip_path, where = Path(args.zip), Path(args.pdf) if args.pdf else None
     try:
-        project, pdf = bundle.read(
-            Path(args.zip), Path(args.pdf) if args.pdf else None, force=args.force
-        )
+        cuts = bundle.has_cuts(zip_path)
+    except (OSError, ValueError) as error:
+        print(f"cannot open: {error}")
+        return 1
+
+    if not cuts and not args.detect:
+        print(f"{zip_path.name} carries no cuts — it was written by a producer that")
+        print("does not record them. Its PDF can be cut again from scratch, but that")
+        print("is a fresh session: the cuts will be detection's, and the markers land")
+        print("only if detection happens to find the same number of slices.")
+        if not _confirm("Open it that way?"):
+            return 1
+
+    try:
+        project, pdf = bundle.read(zip_path, where, force=args.force, detect=not cuts)
     except (ValueError, KeyError) as error:
         print(f"cannot open: {error}")
         return 1
 
     saved = project.save()
-    print(f"{pdf.name}: {len(project.pages)} pages, {len(project.kept_slices())} slices")
+    kept = project.kept_slices()
+    print(f"{pdf.name}: {len(project.pages)} pages, {len(kept)} slices")
+    if not cuts:
+        marked = sum(len(m) for page in project.pages for m in page.markers)
+        print("  cut from scratch by detection — check every cut before exporting")
+        print(
+            f"  {marked} markers placed by position"
+            if marked
+            else "  markers not placed: detection found a different number of slices"
+        )
     print(f"  project written to {saved.name}")
     if args.no_edit:
         return 0
@@ -182,6 +212,11 @@ def main(argv: list[str] | None = None) -> int:
     opn.add_argument(
         "--force", action="store_true", help="overwrite an existing PDF or project file"
     )
+    opn.add_argument(
+        "--detect",
+        action="store_true",
+        help="for a bundle with no cuts: cut its PDF from scratch, without asking",
+    )
     opn.set_defaults(func=_open)
 
     ed = sub.add_parser("edit", help="open the editor")