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/engrave.py

@@ -14,7 +14,7 @@ from __future__ import annotations
 import cv2
 import numpy as np
 from PySide6.QtCore import Qt
-from PySide6.QtGui import QImage, QKeySequence, QPixmap, QShortcut
+from PySide6.QtGui import QImage, QIntValidator, QKeySequence, QPixmap, QShortcut
 from PySide6.QtWidgets import (
     QCheckBox,
     QComboBox,
@@ -33,6 +33,7 @@ from PySide6.QtWidgets import (
 
 from . import lilypond
 from .detect import staff_height
+from .editor import section
 from .project import Project, Replacement, Voice
 
 
@@ -180,6 +181,16 @@ class EngraveWindow(QDialog):
         self.print_time.toggled.connect(self._settings_changed)
         row.addWidget(self.print_time, 1)
         top.addRow("Time", row)
+
+        # Per slice, unlike key and time: which measure a system starts at is
+        # the one thing that changes with every slice and cannot be inherited.
+        self.bar = QLineEdit("" if self.replacement.bar is None else str(self.replacement.bar))
+        self.bar.setValidator(QIntValidator(1, 9999, self.bar))
+        self.bar.setFixedWidth(70)
+        self.bar.setPlaceholderText("none")
+        self.bar.setToolTip("Printed above the first bar, as a printed score numbers its systems")
+        self.bar.textChanged.connect(self._bar_changed)
+        top.addRow("First bar", self.bar)
         box.addLayout(top)
 
         voices_label = QLabel("Voices")
@@ -209,11 +220,15 @@ class EngraveWindow(QDialog):
         self.status.setWordWrap(True)
         box.addWidget(self.status)
 
+        # Collapsed: the source is what the form writes for you, so it is for
+        # checking what a field did, not for working in. Open it and it stays
+        # open for the life of the window.
+        raw = section("LilyPond source", box, expanded=False)
         self.generated = QPlainTextEdit()
         self.generated.setReadOnly(True)
-        self.generated.setMaximumHeight(120)
+        self.generated.setMaximumHeight(220)
         self.generated.setStyleSheet("color: #808080;")
-        box.addWidget(self.generated)
+        raw.addWidget(self.generated)
         self._refresh_source()
         return panel
 
@@ -240,6 +255,10 @@ class EngraveWindow(QDialog):
         row.setParent(None)
         self._refresh_source()
 
+    def _bar_changed(self, text: str) -> None:
+        self.replacement.bar = int(text) if text.strip().isdigit() else None
+        self._refresh_source()
+
     def _settings_changed(self) -> None:
         # Set on the song, not the slice: they are song properties in practice,
         # and this is what makes the next re-engraved slice open pre-filled.