Add package skeleton, source classification and raster loading

The pdf module is the whole of M1 except the debug overlay, which needs
detection results to draw.

Source type is detected per PDF by looking for a page-covering image,
and is always reported for confirmation rather than applied silently
(ADR 0004). Rasters are extracted via Pixmap(doc, xref) rather than by
decoding extract_image() bytes, because MuPDF handles JBIG2 and CCITT
scans that no image library will.

Scanned pages are extracted at the embedded image's native resolution;
only vector pages are rendered, at 600 DPI. Verified against the corpus:
Elaman nalka (vector) renders 4959x7017, Ketun joululaulu (scan) loads
1653x2332, Engel (scan) 2552x3504 — and Engel's page 2 is 2480 wide
where page 1 is 2552, so scan width varies within one PDF.

tests/test_pdf.py builds its own PDFs so the check runs without corpus
files, which are copyrighted and gitignored.

Closes #1, #2, #3

noteman_slicer/cli.py

@@ -0,0 +1,45 @@
+"""Command line entry point."""
+
+from __future__ import annotations
+
+import argparse
+import sys
+
+from . import __version__
+from .pdf import SourceType, open_source, page_raster
+
+
+def _info(args: argparse.Namespace) -> int:
+    source = open_source(args.pdf, SourceType(args.type) if args.type else None)
+    note = f" (detected {source.detected.value}, overridden)" if source.overridden else ""
+    print(f"{source.path.name}: {source.type.value}{note}, {len(source)} pages")
+    for i in range(len(source)):
+        h, w = page_raster(source, i).shape
+        print(f"  p{i + 1:<3} {w}x{h}")
+    source.close()
+    return 0
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(
+        prog="noteman-slicer",
+        description="Cut score PDFs into noteman's slice images and markers.",
+    )
+    parser.add_argument("--version", action="version", version=__version__)
+    sub = parser.add_subparsers(dest="command", required=True)
+
+    info = sub.add_parser("info", help="classify a PDF and report its page rasters")
+    info.add_argument("pdf")
+    info.add_argument(
+        "--type",
+        choices=[t.value for t in SourceType],
+        help="override source-type detection",
+    )
+    info.set_defaults(func=_info)
+
+    args = parser.parse_args(argv)
+    return args.func(args)
+
+
+if __name__ == "__main__":
+    sys.exit(main())