Function renderKeySignature

Render a key signature to braille music notation.

Per Music Braille Code 2015, Table 6:

• 1–3 accidentals are written out as that many accidental signs: 2 sharps → ⠩⠩, 3 flats → ⠣⠣⠣
• 4 or more use numeric form: number sign + upper-cell digit + accidental: 4 sharps → ⠼⠙⠩, 5 flats → ⠼⠑⠣
• C major / A minor (key=0) → empty (no key signature)