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.

tests/test_markers.py

@@ -165,6 +165,38 @@ def main() -> int:
     assert back.bars[second] == 7
     assert back.replacements[second].voices[0].lyrics == "la la"
 
+    # A bundle from a producer that records no cuts: refused by default, and
+    # cut from scratch by detection when the caller says so. The slices are in
+    # reading order either way, so markers can be lined up by position — but
+    # only when detection finds exactly as many.
+    plain = tmp / "plain.zip"
+    with zipfile.ZipFile(out) as src, zipfile.ZipFile(plain, "w") as dst:
+        for name in src.namelist():
+            data = src.read(name)
+            if name == "song.json":
+                manifest = json.loads(data)
+                manifest.pop("source")
+                for entry in manifest["slices"]:
+                    entry.pop("page", None)
+                    entry.pop("slot", None)
+                data = json.dumps(manifest).encode()
+            dst.writestr(name, data)
+    assert bundle.has_cuts(out) and not bundle.has_cuts(plain)
+    try:
+        bundle.read(plain, tmp / "nocuts.pdf")
+    except bundle.NoCuts as error:
+        assert "no cuts" in str(error), error
+    else:
+        raise AssertionError("a bundle without cuts should not open silently")
+
+    cut_again, again_pdf = bundle.read(plain, tmp / "nocuts.pdf", detect=True)
+    assert again_pdf.exists()
+    assert cut_again.metadata["title"] == "Test song", "the title block still comes back"
+    assert len(cut_again.kept_slices()) == len(reloaded.kept_slices())
+    placed = [m for page in cut_again.pages for slot in page.markers for m in slot]
+    assert len(placed) == 3, placed
+    assert placed[0].label == "A"
+
     # Unpacking never lands on files that are already there.
     try:
         bundle.read(out, tmp / "reopened.pdf")
@@ -174,7 +206,17 @@ def main() -> int:
         raise AssertionError("reopening over an existing PDF should be refused")
 
     source.close()
-    for f in (pdf, out, saved, unpacked, default_path(unpacked), default_path(pdf)):
+    for f in (
+        pdf,
+        out,
+        plain,
+        saved,
+        unpacked,
+        again_pdf,
+        default_path(unpacked),
+        default_path(again_pdf),
+        default_path(pdf),
+    ):
         f.unlink(missing_ok=True)
     tmp.rmdir()
     print("ok")