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/project.py

@@ -113,6 +113,11 @@ class Project:
     levels: tuple[int, int] = (0, 255)
     metadata: dict[str, str] = field(default_factory=dict)
     path: Path | None = None
+    # Set once the song has been exported. A project is spent at that point:
+    # opening the PDF again starts a fresh session from detection rather than
+    # resuming, so a re-cut never begins from stale decisions. `--resume`
+    # overrides it when the old state really is wanted.
+    exported: bool = False
 
     # -- geometry helpers -------------------------------------------------
 
@@ -184,6 +189,7 @@ class Project:
             "v": FORMAT_VERSION,
             "source": self.source.name,
             "source_hash": self.source_hash,
+            "exported": self.exported,
             "content_rect": list(self.content_rect),
             "levels": list(self.levels),
             "metadata": self.metadata,
@@ -229,6 +235,7 @@ class Project:
             levels=tuple(data["levels"]),
             metadata=data.get("metadata", {}),
             path=path,
+            exported=data.get("exported", False),
         )
 
     def source_changed(self) -> bool:
@@ -236,6 +243,30 @@ class Project:
         return self.source.exists() and hash_file(self.source) != self.source_hash
 
 
+def open_project(source, *, resume: bool = False) -> Project:
+    """The project for a PDF: resumed, or a fresh session from detection.
+
+    A project that has been exported is spent. Opening the PDF again starts
+    over from detection rather than resuming, so a re-cut never inherits stale
+    decisions. `resume` overrides that when the old state really is wanted.
+    """
+    from .detect import detect_page
+    from .pdf import page_raster
+
+    path = default_path(source.path)
+    if path.exists():
+        existing = Project.load(path)
+        if resume or not existing.exported:
+            return existing
+
+    detections, heights = [], []
+    for i in range(len(source)):
+        gray = page_raster(source, i)
+        detections.append(detect_page(gray))
+        heights.append(gray.shape[0])
+    return Project.from_detection(source.path, detections, heights)
+
+
 def default_path(source: Path) -> Path:
     return Path(source).with_suffix(SUFFIX)