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.

tests/test_render.py

@@ -16,7 +16,7 @@ sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
 from noteman_slicer import bundle  # noqa: E402
 from noteman_slicer.detect import detect_page  # noqa: E402
 from noteman_slicer.pdf import open_source, page_raster  # noqa: E402
-from noteman_slicer.project import Cut, Project  # noqa: E402
+from noteman_slicer.project import Cut, Project, default_path  # noqa: E402
 from noteman_slicer.render import (  # noqa: E402
     ALPHA_LEVELS,
     apply_levels,
@@ -151,8 +151,9 @@ def main() -> int:
         assert all(f in names for f in files)
 
     source.close()
-    for f in (pdf, out):
-        f.unlink()
+    # Exporting marks the project spent, which writes the project file.
+    for f in (pdf, out, default_path(pdf)):
+        f.unlink(missing_ok=True)
     tmp.rmdir()
     print("ok")
     return 0