Add markers: placement, labels and click-to-pick jump targets

Markers are stored per (page, slot), parallel to the discard flags, so
adding or removing a cut keeps them aligned with their slices. On a
split they stay with the upper half: a marker sits on a printed symbol
and nothing can say which side that symbol landed on, so predictable
beats clever.

Jump targets are chosen by clicking the slice rather than from the
thumbnail strip the plan called for. Less code, and it reads the score
instead of a list of thumbnails - which is what you want when hunting
for the Coda sign. Any page; PageUp/PageDown while picking.

Export resolves (page, slot) to the bundle's array index, the only
cross-reference the format has. A jump whose target was discarded or
re-cut away is dropped rather than exported dangling, since noteman
would have nothing to resolve it to.

tests/test_markers.py covers the enum size - that is the coupling
between two repos - along with cut-edit alignment, index resolution,
the dangling-target drop, and round-trips through both the project file
and a real bundle.

Closes #28
Closes #29
Closes #30

noteman_slicer/bundle.py

@@ -39,7 +39,35 @@ def song_json(project: Project, files: list[str]) -> dict:
         value = project.metadata.get(field)
         if value:
             payload[field] = value
-    payload["slices"] = [{"file": name} for name in files]
+
+    kept = project.kept_slices()
+    # Markers reference slices by (page, slot) while editing, because that is
+    # what survives adding and removing cuts. In the bundle they become the
+    # array index, which is the only cross-reference the format has.
+    index_of = {position: i for i, position in enumerate(kept)}
+
+    slices: list[dict] = []
+    for name, (page, slot) in zip(files, kept):
+        entry: dict = {"file": name}
+        markers = []
+        for marker in project.pages[page].markers[slot]:
+            item: dict = {"type": marker.type}
+            if marker.label:
+                item["label"] = marker.label
+            if marker.destination is not None:
+                target = index_of.get(tuple(marker.destination))
+                # A jump whose target was discarded or re-cut away is dropped
+                # rather than exported dangling: noteman would have nothing to
+                # resolve it to.
+                if target is None:
+                    continue
+                item["destination"] = target
+            markers.append(item)
+        if markers:
+            entry["markers"] = markers
+        slices.append(entry)
+
+    payload["slices"] = slices
     return payload