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_project.py

@@ -67,6 +67,12 @@ def main() -> int:
     assert reloaded.source_hash == project.source_hash
     assert not reloaded.source_changed()
 
+    # A project is spent once exported: reopening starts fresh.
+    assert not reloaded.exported
+    reloaded.exported = True
+    reloaded.save()
+    assert Project.load(saved).exported
+
     # A PDF edited underneath must be reported, not silently re-cut.
     pdf.write_bytes(b"%PDF-1.7 different bytes entirely")
     assert reloaded.source_changed()