Add the render pipeline and bundle export

Project state plus PDF in, finished slice images out. Slices are cut as
polygons rather than row ranges, so a stepped cut yields a slice with a
transparent notch instead of one that covers its neighbour.

Masking paints white, which the ink-to-alpha step turns into full
transparency — the same outcome the spec asks for, one step earlier.

Scale normalises every slice to the median staff height before fitting
the song to 1920px, so a rescanned page sits at the same note size as
its neighbours. The cap only ever shrinks: a song narrower than 1920
stays narrower.

Alpha quantisation rounds to 16 values spanning 0-255 inclusive.
Flooring, as first written, capped full ink at 240 and left every note
6% transparent — caught by decoding an exported slice rather than by
reading the code.

Ketun joululaulu exports 24 slices at a uniform 1489px, under the cap
and correctly not upscaled from its 200 DPI source; Feliz Navidad 20;
Elaman nalka 18.

Closes #21
Closes #22
Closes #23
Closes #24
Closes #25
Closes #27

noteman_slicer/cli.py

@@ -82,6 +82,35 @@ def _project(args: argparse.Namespace) -> int:
     return 0
 
 
+def _export(args: argparse.Namespace) -> int:
+    from . import bundle
+    from .project import Project, default_path
+
+    source = open_source(args.pdf, SourceType(args.type) if args.type else None)
+    path = default_path(source.path)
+
+    if path.exists():
+        project = Project.load(path)
+        if project.source_changed():
+            print("WARNING: the PDF has changed since these cuts were made")
+    else:
+        detections, heights = [], []
+        for i in range(len(source)):
+            gray = page_raster(source, i)
+            detections.append(detect_page(gray))
+            heights.append(gray.shape[0])
+        project = Project.from_detection(source.path, detections, heights)
+        print("no project file; exporting straight from detection")
+
+    out = Path(args.out) if args.out else source.path.with_suffix(".zip")
+    bundle.write(project, source, out)
+    size = out.stat().st_size
+    slices = len(project.kept_slices())
+    print(f"{out}  {slices} slices, {size / 1024:.0f} KB ({size / max(slices, 1) / 1024:.1f} KB/slice)")
+    source.close()
+    return 0
+
+
 def main(argv: list[str] | None = None) -> int:
     parser = argparse.ArgumentParser(
         prog="noteman-slicer",
@@ -113,6 +142,12 @@ def main(argv: list[str] | None = None) -> int:
     proj.add_argument("--type", choices=[t.value for t in SourceType])
     proj.set_defaults(func=_project)
 
+    exp = sub.add_parser("export", help="render the song and write a bundle")
+    exp.add_argument("pdf")
+    exp.add_argument("--out", help="output zip (default: alongside the PDF)")
+    exp.add_argument("--type", choices=[t.value for t in SourceType])
+    exp.set_defaults(func=_export)
+
     args = parser.parse_args(argv)
     return args.func(args)