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_editor.py

@@ -85,11 +85,15 @@ def main() -> int:
     editor.metadata["composer"].setText("trad.")
     assert project.metadata["title"] == "Ketun joululaulu"
 
-    # Content rectangle edits and reset.
+    # Content rectangle edits, and reset going back to detection's proposal for
+    # the page as it now stands — not to the whole page, which would undo the
+    # thing the rectangle exists for.
+    expected = detect_page(editor._preview(0), skew=0.0).content
     page.content_rect = (0.05, 0.02, 0.95, 0.98)
     assert project.page_content_rect(0) == (0.05, 0.02, 0.95, 0.98)
     editor._reset_rect()
-    assert project.page_content_rect(0) == project.content_rect
+    assert project.page_content_rect(0) == expected, (project.page_content_rect(0), expected)
+    assert project.page_content_rect(0) != (0.0, 0.0, 1.0, 1.0)
 
     # Autosave target, then a round-trip through disk.
     editor._save()