fix: fall back to the Fallback face for glyphs HarfBuzz can't find
draw_shaped_run shaped every run against the primary style face only and
looked glyphs up via font.get_shaped, which (by design, per its doc
comment) never consults font.Style.Fallback. A codepoint missing from
the primary font shaped to HarfBuzz's .notdef (glyph_index 0) and was
silently dropped instead of falling through to the fallback face —
unlike st, which resolves a fallback font per missing glyph.
Route the glyph_index == 0 case through the rune-keyed font.get instead,
which already has the Fallback-face fallback logic but was previously
unreachable from the render path.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>