Add the editor: page view, cut editing, discard, levels, metadata

QGraphicsView for the viewport, with cuts and the content rectangle
manipulated by hit-testing in the view rather than as movable items —
the geometry is normalised, so what the screen shows and what the
renderer uses are the same numbers at a different zoom.

Cuts are edited as polylines: double-click adds one, drag moves it,
Ctrl-click inserts a vertex, right-click deletes a vertex or the whole
cut. That is how a straight cut becomes the stepped cut Engel needs.

Slice boundaries are drawn, not just cut lines, so a trim anomaly is
visible before export rather than after. Discarded slices are shaded.

Pages preview at 1800px regardless of source resolution, cached per
page, because re-reading a 4959x7017 vector render on every slider move
is unusable.

Autosave is debounced at 800ms and also fires on close.

Closes #12
Closes #14
Closes #15
Closes #16
Closes #17
Closes #18
Closes #19
Closes #20
Closes #26

noteman_slicer/cli.py

@@ -111,6 +111,12 @@ def _export(args: argparse.Namespace) -> int:
     return 0
 
 
+def _edit(args: argparse.Namespace) -> int:
+    from .editor import launch
+
+    return launch(Path(args.pdf), SourceType(args.type) if args.type else None)
+
+
 def main(argv: list[str] | None = None) -> int:
     parser = argparse.ArgumentParser(
         prog="noteman-slicer",
@@ -148,6 +154,11 @@ def main(argv: list[str] | None = None) -> int:
     exp.add_argument("--type", choices=[t.value for t in SourceType])
     exp.set_defaults(func=_export)
 
+    ed = sub.add_parser("edit", help="open the editor")
+    ed.add_argument("pdf")
+    ed.add_argument("--type", choices=[t.value for t in SourceType])
+    ed.set_defaults(func=_edit)
+
     args = parser.parse_args(argv)
     return args.func(args)