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_markers.py

@@ -98,14 +98,15 @@ def main() -> int:
     # from the bundle alone.
     project.key, project.time = "aes", "3/4"
     page.replacements[second] = Replacement(
-        voices=[Voice("treble", "c4 d e f", "la la la la"), Voice("bass", " c4 d e f ", "  ")]
+        voices=[Voice("treble", "c4 d e f", "la la la la"), Voice("bass", " c4 d e f ", "  ")],
+        bar=33,
     )
     engraved = song_json(project, names)["slices"]
     assert "engraving" not in engraved[0], "a scanned slice has no notation"
     ly = engraved[1]["engraving"]
     assert ly["lang"] == "lilypond"
     # Song defaults are resolved per slice: reading one slice needs no context.
-    assert (ly["key"], ly["time"], ly["print_time"]) == ("aes", "3/4", False)
+    assert (ly["key"], ly["time"], ly["print_time"], ly["bar"]) == ("aes", "3/4", False, 33)
     assert ly["voices"][0] == {"clef": "treble", "notes": "c4 d e f", "lyrics": "la la la la"}
     assert "lyrics" not in ly["voices"][1], "an empty field is absent, not empty"
     assert ly["voices"][1]["notes"] == "c4 d e f"