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/editor.py

@@ -48,7 +48,7 @@ from . import bundle
 from .bundle import METADATA_FIELDS
 from .detect import deskew, detect_page
 from .pdf import Source, open_source, page_raster
-from .project import Cut, Project, default_path
+from .project import Cut, Project, open_project
 from .render import apply_levels
 
 PREVIEW_MAX = 1800  # display resolution; geometry stays normalised
@@ -356,17 +356,10 @@ class Editor(QMainWindow):
         discard = QPushButton("Toggle discard  (D)")
         discard.clicked.connect(self.view.toggle_discard)
         form.addRow(discard)
-        rect_row = QHBoxLayout()
-        fit = QPushButton("Auto-fit")
-        fit.setToolTip("Propose the content rectangle from the staff lines on this page")
-        fit.clicked.connect(lambda: self._autofit_rect(False))
-        fit_all = QPushButton("Auto-fit all")
-        fit_all.clicked.connect(lambda: self._autofit_rect(True))
-        reset = QPushButton("Reset")
+        reset = QPushButton("Reset content rectangle")
+        reset.setToolTip("Back to the rectangle detection proposed for this page")
         reset.clicked.connect(self._reset_rect)
-        for button in (fit, fit_all, reset):
-            rect_row.addWidget(button)
-        form.addRow("Content rect", rect_row)
+        form.addRow(reset)
         box.addWidget(page_box)
 
         meta_box = QGroupBox("Song")
@@ -482,25 +475,16 @@ class Editor(QMainWindow):
         }
         self.autosave.start()
 
-    def _autofit_rect(self, every_page: bool) -> None:
-        """Re-propose the content rectangle from the staff lines.
-
-        Detection does this for new projects; this is how a project made before
-        it existed, or one whose rectangle was dragged wrong, gets it back.
-        """
-        pages = range(len(self.project.pages)) if every_page else [self.index]
-        for i in pages:
-            # The preview is already deskewed, so the sweep is skipped.
-            proposal = detect_page(self._preview(i), skew=0.0)
-            self.project.pages[i].content_rect = proposal.content
-        self.view.redraw()
-        self._touched()
-        self.statusBar().showMessage(
-            f"content rectangle fitted on {len(list(pages))} page(s)", 2000
-        )
-
     def _reset_rect(self) -> None:
-        self.project.pages[self.index].content_rect = None
+        """Back to what detection proposed for this page.
+
+        Not to the whole page: the proposal is what excludes the scan-edge
+        junk, so clearing to full width would undo the thing the rectangle
+        exists for. The preview is already deskewed, so the sweep is skipped.
+        """
+        self.project.pages[self.index].content_rect = detect_page(
+            self._preview(self.index), skew=0.0
+        ).content
         self.view.redraw()
         self._touched()
 
@@ -524,7 +508,9 @@ class Editor(QMainWindow):
         QMessageBox.information(
             self,
             "Exported",
-            f"{out.name}\n{len(self.project.kept_slices())} slices, {size:.0f} KB",
+            f"{out.name}\n{len(self.project.kept_slices())} slices, {size:.0f} KB\n\n"
+            "This project is now spent — opening the PDF again starts a fresh "
+            "session from detection.",
         )
 
     def closeEvent(self, event) -> None:
@@ -532,27 +518,20 @@ class Editor(QMainWindow):
         super().closeEvent(event)
 
 
-def launch(pdf: Path, source_type=None) -> int:
+def launch(pdf: Path, source_type=None, resume: bool = False) -> int:
     app = QApplication(sys.argv[:1])
     source = open_source(pdf, source_type)
-    path = default_path(source.path)
 
-    if path.exists():
-        project = Project.load(path)
-        if project.source_changed():
-            QMessageBox.warning(
-                None,
-                "Source changed",
-                "The PDF has changed since these cuts were made.\n"
-                "Cuts may no longer line up with the music.",
-            )
-    else:
-        detections, heights = [], []
-        for i in range(len(source)):
-            gray = page_raster(source, i)
-            detections.append(detect_page(gray))
-            heights.append(gray.shape[0])
-        project = Project.from_detection(source.path, detections, heights)
+    # An exported project is spent: this opens a fresh session from detection
+    # rather than resuming decisions that have already been shipped.
+    project = open_project(source, resume=resume)
+    if project.path is not None and project.source_changed():
+        QMessageBox.warning(
+            None,
+            "Source changed",
+            "The PDF has changed since these cuts were made.\n"
+            "Cuts may no longer line up with the music.",
+        )
 
     window = Editor(source, project)
     window.resize(1500, 950)