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.

noteman_slicer/bundle.py

@@ -69,6 +69,7 @@ def _engraving(project: Project, page: int, slot: int) -> dict | None:
         "key": replacement.key or project.key,
         "time": replacement.time or project.time,
         "print_time": replacement.print_time,
+        **({"bar": replacement.bar} if replacement.bar else {}),
         "voices": [
             {"clef": v.clef, "notes": v.notes.strip()}
             | ({"lyrics": v.lyrics.strip()} if v.lyrics.strip() else {})