macOS's Geeza Pro is an AAT font: its morx shaping takes no context from
outside the run, so different_style_arabic_boundary_receives_joining_context
failed on Macs (reported in #610). The tests now use the first candidate
with a GSUB table and skip when there is none.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>