Gets the Unicode text the character code stands for, worked out by Font when read: one or more UTF-16 units (a ligature, a surrogate pair), empty for a code mapped to no text, U+FFFD when nothing maps it.
public ReadOnlySpan<char> Unicode { get; }
ISO 32000-2 §9.10.2; see GetUnicode(CharacterCode, Span<Char>, out UnicodeSource). The span lives in a buffer of the calling thread: it is valid until Unicode is read again on that thread, so copy it to keep it. Allocates nothing once the font's tables are built.