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.

CONTEXT.md

@@ -51,9 +51,12 @@ two repos.
 **Project**:
 The persistent state of slicing one song: the source PDF it points at, its cuts,
 discards, content rectangle, levels, staff-height overrides, markers and
-metadata. Autosaved beside the PDF; the bundle is generated from it, so any
-export can be regenerated without repeating human work. One PDF, one song, one
-project, one bundle — never a many-to-one in any direction.
+metadata. Autosaved beside the PDF; the bundle is generated from it. One PDF,
+one song, one project, one bundle — never a many-to-one in any direction.
+
+**Spent** once its song has been exported: opening the PDF again begins a fresh
+session from detection rather than resuming, so a re-cut never inherits
+decisions that have already shipped.
 _Avoid_: session, document, edit list
 
 **Bundle**: