Add LilyPond slice replacement with a structured engrave window

Re-engraving is a rescue path for the handful of systems a scan cannot
deliver, so the window is an editing surface rather than an automation
project. Three full-width rows - the scanned system, the render, the
form - because a system is wide and short and the job is comparing one
against the other bar by bar. The render is shown scaled to the scan's
staff height, which is what export does anyway, so it previews the real
thing.

A form rather than a text box. Key and time are slice-level, clef,
notes and lyrics per voice: every staff in a system carries the same key
signature, and Kaipaava proves it across five-staff and two-staff
systems alike. Notes and lyrics stay raw LilyPond, so slurs, dynamics,
tuplets and the laissezVibrer/repeatTie idiom for ties crossing into the
next slice all work untouched.

Notes are entered in \relative mode, referenced to the middle of each
clef's staff, so a part needs no octave marks at all in the common case.

The time signature is used for spacing and bar checks but not printed:
the printed score repeats the key at every system and the time only at
the first, so a re-engraved middle slice showing one would stand out.

Seeded from what can be known reliably. Voice count comes from counting
staves in the slice; key, time and clefs are inherited from the song,
because the slices being re-engraved are the illegible ones and reading
a key signature off them is exactly the measurement that fails. After
the first replacement in a song only the notes need typing.

Staff counting needed two corrections against the corpus: compare gaps
against line spacing rather than staff height, since adjacent staves can
sit closer together than one staff is tall; and require five lines in a
group, since Engel's 'uh______' lyric extenders are long horizontal runs
too and each counted as a staff. Kaipaava now reads 2,2,2,2,5 on page 1,
Ketun 6, Engel 4.

Also in this change:

- Title is required for export, every other metadata field optional,
  enforced in bundle.write so the CLI and the editor both get it. Tempo
  added; noteman already has a free-form column for it.
- The panel is a splitter rather than a fixed width, sections collapse
  under bold grey disclosure headers, and it scrolls.
- A re-engraved slice is washed amber with an ENGRAVED badge, and
  markers get badges too. Thin coloured text was invisible against a
  scan.

Closes #31
Closes #32
Closes #33
Closes #34

noteman_slicer/bundle.py

@@ -29,6 +29,9 @@ METADATA_FIELDS = (
     "arranger",
     "lyricist",
     "translator",
+    # Free-form, matching noteman's own column: scores notate tempo as a mix of
+    # BPM ("♩=72"), Italian ("Andante") and prose.
+    "tempo",
     "voices",
 )
 
@@ -72,8 +75,18 @@ def song_json(project: Project, files: list[str]) -> dict:
 
 
 def write(project: Project, source: Source, path: Path) -> Path:
-    """Render the song and write the bundle. Returns the zip path."""
+    """Render the song and write the bundle. Returns the zip path.
+
+    A title is required; every other metadata field is optional. noteman's own
+    rule is that a song needs a title and at least one slice, and a bundle that
+    cannot become a song is not worth writing.
+    """
+    if not project.metadata.get("title", "").strip():
+        raise ValueError("a title is required before a song can be exported")
+
     images = render_song(project, source)
+    if not images:
+        raise ValueError("no slices to export — every slice is discarded")
     names = [f"{i + 1:03}.webp" for i in range(len(images))]
 
     path = Path(path)