Addresses #16584
Problem: TUI word-wise cursor movement treated entire CJK runs as a
single word, so Option/Alt+Left and Right skipped too far when editing
East Asian text.
Solution: Use Unicode word-boundary segments within each non-whitespace
run so CJK text advances one segment at a time while preserving
separator and delete-word behavior, and add regression coverage for CJK
and mixed-script navigation.
Testing: Manually tested solution by pasting text that includes CJK
characters into the composer and confirmed that keyboard navigation
worked correctly (after confirming it didn't prior to the change).
Eric Traut
82b061afb2