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.

docs/spec.md

@@ -263,9 +263,19 @@ discards, content rectangle, skew angles, levels, staff-height overrides, marker
 and metadata. The bundle is *generated* from it, so export is a pure function of
 the project file plus the PDF.
 
-It buys crash safety, resume across sessions (authoring is trickle-in), and
-**re-export** — change the 1920 cap, fix one cut, or add the SVG renderer later,
-and every song's bundle regenerates without repeating any human work.
+It buys crash safety and resume across sessions, since authoring is trickle-in:
+a session interrupted halfway through a 12-page scan picks up exactly where it
+stopped.
+
+**A project is spent once its song has been exported.** Export records that in
+the file, and opening the PDF again starts a *fresh session from detection*
+rather than resuming. A re-cut therefore never inherits decisions that have
+already shipped. `--resume` overrides it on the `edit`, `export` and `project`
+commands when the old state really is wanted.
+
+The cost is deliberate: re-export is no longer free. Changing the width cap or
+adding the SVG renderer later means re-cutting each song by hand rather than
+regenerating every bundle from its project file.
 
 The project file references the PDF and never contains it; the hash lets the
 editor warn if the PDF changed underneath.