HarfBuzz shaping backend for Typf.
Shaping is the step that turns characters into positioned glyphs. That work is simple for plain Latin text, but it becomes essential for Arabic joins, Devanagari conjuncts, Thai vowel placement, and any script where one character does not map cleanly to one painted glyph. This crate delegates that work to HarfBuzz and translates Typf's neutral API into HarfBuzz calls.