Treat a project as spent once its song has been exported

Opening an exported song starts a fresh session from detection instead
of resuming: cuts, discards and metadata do not carry over, so a re-cut
never inherits decisions that have already shipped. --resume overrides
it on edit, export and project.

This reverses what was agreed in planning and written into docs/spec.md
and CONTEXT.md, which promised resume-across-sessions and re-export.
Both are corrected. The cost is deliberate and worth stating: changing
the width cap or adding the SVG renderer later now means re-cutting each
song by hand rather than regenerating every bundle from its project
file.

Export records the flag in bundle.write, so no caller can forget it.

Also removed --refit and the Auto-fit buttons, which were added without
being asked for and whose only purpose - migrating projects made before
the content rectangle was proposed - disappears once exported projects
start fresh. Reset now restores detection's proposal rather than the
whole page: clearing to full width would undo the thing the rectangle
exists for, so one button covers it.

open_project() replaces four copies of load-or-detect across the CLI
and the editor.

noteman_slicer/cli.py

@@ -51,32 +51,18 @@ def _detect(args: argparse.Namespace) -> int:
 
 
 def _project(args: argparse.Namespace) -> int:
-    from .project import Project, default_path
+    from .project import default_path, open_project
 
     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")
+    project = open_project(source, resume=args.resume and not args.force)
+    if project.path is None:
+        print(f"{path.name}: fresh session from detection")
+    else:
+        print(f"{path.name}: resumed")
         if project.source_changed():
             print("  WARNING: the PDF has changed since these cuts were made")
-        if args.refit:
-            # Re-propose the content rectangle without disturbing cuts,
-            # discards or metadata — for projects made before detection
-            # proposed one, or whose rectangle was dragged wrong.
-            for i in range(len(project.pages)):
-                gray = page_raster(source, i)
-                project.pages[i].content_rect = detect_page(gray).content
-            print(f"  content rectangle re-fitted on {len(project.pages)} pages")
-    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):
@@ -92,23 +78,14 @@ def _project(args: argparse.Namespace) -> int:
 
 def _export(args: argparse.Namespace) -> int:
     from . import bundle
-    from .project import Project, default_path
+    from .project import open_project
 
     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")
+    project = open_project(source, resume=args.resume)
+    if project.path is None:
+        print("no unspent project state; exporting straight from detection")
+    elif project.source_changed():
+        print("WARNING: the PDF has changed since these cuts were made")
 
     out = Path(args.out) if args.out else source.path.with_suffix(".zip")
     bundle.write(project, source, out)
@@ -122,7 +99,9 @@ def _export(args: argparse.Namespace) -> int:
 def _edit(args: argparse.Namespace) -> int:
     from .editor import launch
 
-    return launch(Path(args.pdf), SourceType(args.type) if args.type else None)
+    return launch(
+        Path(args.pdf), SourceType(args.type) if args.type else None, resume=args.resume
+    )
 
 
 def main(argv: list[str] | None = None) -> int:
@@ -154,9 +133,9 @@ def main(argv: list[str] | None = None) -> int:
     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(
-        "--refit",
+        "--resume",
         action="store_true",
-        help="re-propose the content rectangle, keeping cuts and metadata",
+        help="reopen an already-exported project instead of starting fresh",
     )
     proj.add_argument("--type", choices=[t.value for t in SourceType])
     proj.set_defaults(func=_project)
@@ -164,11 +143,17 @@ def main(argv: list[str] | None = None) -> int:
     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("--resume", action="store_true", help="use already-exported project state")
     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(
+        "--resume",
+        action="store_true",
+        help="reopen an already-exported project instead of starting fresh",
+    )
     ed.add_argument("--type", choices=[t.value for t in SourceType])
     ed.set_defaults(func=_edit)