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/bundle.py

@@ -62,4 +62,10 @@ def write(project: Project, source: Source, path: Path) -> Path:
             zf.write(project.source, "original.pdf")
         for name, data in zip(names, images):
             zf.writestr(name, data, zipfile.ZIP_STORED)
+
+    # The project is spent once its song has been exported: the next edit
+    # session starts fresh from detection rather than resuming these decisions.
+    # Recorded here so no caller can forget it.
+    project.exported = True
+    project.save()
     return path