Add project state with cuts, discards and atomic save

Everything the human decides, in normalised page coordinates so the
file is independent of DPI and of which renderer produced it. The
bundle will be generated from this, which is what makes re-export
possible without repeating human work.

Cuts are polylines from the start, two points being the ordinary
straight case, so the stepped cuts Engel needs are a data question
rather than a migration. Adding a cut splits a slice and copies its
discard flag to both halves; removing one merges them.

Boundary cuts belong here rather than in detection: detection emits
cuts only between systems, so a page would have exactly as many slices
as systems, with the header and footer inside the first and last.
Isolating and discarding them is a slicing decision.

Saves are write-then-rename, so a crash mid-save cannot destroy the
previous state. The PDF is hashed, not copied, so an edit underneath is
reported rather than silently re-cut.

Ketun joululaulu now yields 24 kept slices over 12 pages and Feliz
Navidad 20 over 4, with headers and footers discarded on every page.

Closes #10, #11, #13

noteman_slicer/cli.py

@@ -50,6 +50,38 @@ def _detect(args: argparse.Namespace) -> int:
     return 0
 
 
+def _project(args: argparse.Namespace) -> int:
+    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() and not args.force:
+        project = Project.load(path)
+        print(f"{path.name}: loaded")
+        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(f"{path.name}: created from detection")
+
+    kept = project.kept_slices()
+    for i, page in enumerate(project.pages):
+        flags = "".join("." if d else "#" for d in page.discards)
+        print(f"  p{i + 1:<3} skew {page.skew:+.2f}°  {page.slice_count} slices  [{flags}]")
+    print(f"  {len(kept)} slices kept, {sum(p.slice_count for p in project.pages) - len(kept)} discarded")
+
+    if args.save:
+        print(f"  saved to {project.save(path)}")
+    source.close()
+    return 0
+
+
 def main(argv: list[str] | None = None) -> int:
     parser = argparse.ArgumentParser(
         prog="noteman-slicer",
@@ -74,6 +106,13 @@ def main(argv: list[str] | None = None) -> int:
     det.add_argument("--type", choices=[t.value for t in SourceType])
     det.set_defaults(func=_detect)
 
+    proj = sub.add_parser("project", help="create or inspect the project file for a PDF")
+    proj.add_argument("pdf")
+    proj.add_argument("--save", action="store_true", help="write the project file")
+    proj.add_argument("--force", action="store_true", help="re-detect, discarding existing state")
+    proj.add_argument("--type", choices=[t.value for t in SourceType])
+    proj.set_defaults(func=_project)
+
     args = parser.parse_args(argv)
     return args.func(args)