Keyboard braille: extractHeading passes a KeySignature where a TimeSignature is expected (isinstance tested on the container) #1986

Environment: music21 10.5.0, Python 3.14, macOS.

Summary: BrailleGrandSegment.extractHeading (braille/segment.py, the SIGNATURE branch) tests isinstance on the grouping container instead of the element:

try:
    keySignature, timeSignature = useElement[0], useElement[1]
except IndexError:
    if isinstance(useElement, key.KeySignature):   # container, never true
        keySignature = useElement[0]
    else:
        timeSignature = useElement[0]              # a lone KeySignature lands here

useElement is a grouping (list-like), never a KeySignature, so a mid-piece key change with no simultaneous meter change (a SIGNATURE grouping holding only a KeySignature) falls into the else, is passed to transcribeSignatures, which reads m21TimeSignature.symbol (braille/basic.py) → AttributeError: 'KeySignature' object has no attribute 'symbol', aborting the transcription.

Fix sketch: test the element rather than the container: isinstance(useElement[0], key.KeySignature).

Reproduction sketch: a two-part keyboard score with a key change (no meter change) at an interior measure, transcribed via braille.translate.keyboardPartsToBraille.

Related, found while tracing: the warning path of handleArticulations (braille/basic.py) references music21Note.editorial.fingering, which does not exist — the except branch itself raises AttributeError when a fingering fails to transcribe.