Engrave window: bar numbers, and two silent LilyPond faults

A system can now be given the measure it starts at, in a First bar field
beside key and time. Per slice, since it is the one thing about a
replacement that cannot be inherited from the song. Bar numbering is a
Score property, so it is set once on the first staff, and made visible
only at a line beginning — that vector is fussy: #(#f #t #t) also prints
a number mid-system and #(#f #t #f) prints the second bar's rather than
the first's. It travels in the bundle's engraving object as `bar`.

Two things LilyPond 2.24 was quietly refusing to draw:

`\bar ":|"` and the other old repeat names produce nothing at all — no
error, no warning, exit status 0, just a missing repeat that you find on
the tablet. Every book and forum answer still uses them, so translate
them to the modern spellings.

`\clef treble_8` unquoted is not an octavated clef either. It parses as a
plain treble plus a stray "8" markup that lands under the first note, and
the staff then reads an octave off — a tenor line engraved at soprano
pitch. Quote it.

The source pane, which is where either of those would have been visible,
is now a collapsed section at the bottom rather than a permanent slab. It
uses the panel's own disclosure helper, lifted out of Editor so both can
call it.

tests/test_lilypond.py

@@ -52,6 +52,39 @@ def _scan_pdf(path: Path) -> None:
 
 
 def main() -> int:
+    # Bar aliases are string work, so they are checked whether or not LilyPond
+    # is installed. The old repeat names draw nothing at all in 2.24 — silently,
+    # which is how a missing repeat reaches a tablet.
+    aliased = lilypond.generate(
+        Replacement(voices=[Voice("treble", 'c4 d \\bar ":|" e f \\bar "|:" g', "")]), "c", "4/4"
+    )
+    assert '\\bar ":|."' in aliased and '\\bar ".|:"' in aliased, aliased
+    kept = lilypond.generate(
+        Replacement(voices=[Voice("treble", 'c4 \\bar "|." d', "")]), "c", "4/4"
+    )
+    assert '\\bar "|."' in kept, "a name LilyPond still knows is left alone"
+
+    # An octavated clef name must be quoted. Unquoted, `\clef treble_8` is a
+    # plain treble with a stray "8" markup under the first note, an octave off.
+    tenor = lilypond.generate(
+        Replacement(voices=[Voice("treble_8", "c4 d", "")]), "c", "4/4"
+    )
+    assert '\\clef "treble_8"' in tenor, tenor
+
+    # A bar number is set once, on the first staff, since it is a Score
+    # property, and is visible only at a line beginning — one number above the
+    # first bar, as a printed score numbers its systems.
+    numbered = lilypond.generate(
+        Replacement(voices=[Voice("treble", "c4 d", ""), Voice("bass", "c4 d", "")], bar=33),
+        "c",
+        "4/4",
+    )
+    assert numbered.count("currentBarNumber = #33") == 1, numbered
+    assert "break-visibility = #'#(#f #f #t)" in numbered
+    assert "currentBarNumber" not in lilypond.generate(
+        Replacement(voices=[Voice("treble", "c4 d", "")]), "c", "4/4"
+    ), "an unnumbered system prints no number"
+
     if not lilypond.available():
         print("ok (skipped: LilyPond not installed)")
         return 0
@@ -138,6 +171,7 @@ def main() -> int:
     assert page.replacements[second] is SATB
 
     # Round-trip, including the song-level engraving defaults.
+    SATB.bar = 33
     project.key, project.time, project.clefs = "aes", "3/4", ["treble", "bass"]
     saved = project.save()
     reloaded = Project.load(saved)
@@ -146,6 +180,7 @@ def main() -> int:
     assert restored is not None
     assert [v.clef for v in restored.voices] == ["treble", "bass"]
     assert restored.voices[0].lyrics == "la la la la la la"
+    assert restored.bar == 33, "the slice's bar number survives a save"
     assert reloaded.pages[0].replacements[first] is None
 
     source.close()