Carry a re-engraved slice's notation in the bundle

A replaced slice shipped as pixels only, so the LilyPond behind it died
with the project file — and a project is spent once exported. Correcting
one wrong note meant retyping the system.

Slices gain an optional `engraving` object: the language, key, time and
one entry per voice holding the notes and lyrics verbatim. Structured per
staff rather than one blob of source, because that is what both a later
edit and a MIDI render want; song-level key and time are resolved per
slice so reading one needs no context from its neighbours.

Additive and ignorable, so the format stays at version 1.

noteman_slicer/bundle.py

@@ -39,6 +39,30 @@ METADATA_FIELDS = (
 NUMERIC_FIELDS = frozenset({"tempo"})
 
 
+def _engraving(project: Project, page: int, slot: int) -> dict | None:
+    """The notation behind a re-engraved slice, or None for a scanned one.
+
+    The slice image stays the presentation; this is the notation it was made
+    from, carried so the music can be edited again or turned into sound. Key
+    and time are resolved against the song defaults here — a consumer reading
+    one slice should not have to know what the rest of the song inherited.
+    """
+    replacement = project.pages[page].replacements[slot]
+    if not replacement or not replacement.voices:
+        return None
+    return {
+        "lang": "lilypond",
+        "key": replacement.key or project.key,
+        "time": replacement.time or project.time,
+        "print_time": replacement.print_time,
+        "voices": [
+            {"clef": v.clef, "notes": v.notes.strip()}
+            | ({"lyrics": v.lyrics.strip()} if v.lyrics.strip() else {})
+            for v in replacement.voices
+        ],
+    }
+
+
 def song_json(project: Project, files: list[str]) -> dict:
     payload: dict = {"v": FORMAT_VERSION}
     for field in METADATA_FIELDS:
@@ -62,6 +86,9 @@ def song_json(project: Project, files: list[str]) -> dict:
     slices: list[dict] = []
     for name, (page, slot) in zip(files, kept):
         entry: dict = {"file": name}
+        engraving = _engraving(project, page, slot)
+        if engraving:
+            entry["engraving"] = engraving
         markers = []
         for marker in project.pages[page].markers[slot]:
             item: dict = {"type": marker.type}