Handle spacing grapheme extenders U+FF9E-U+FF9F

[waf]

More fixes around #270

U+FF9E/U+FF9F (halfwidth katakana voiced / semi-voiced sound marks) have Grapheme_Cluster_Break = Extend, so StringInfo groups each into its preceding kana (ﾊ+ﾟ = one cluster). But unlike a normal combining mark that overlays its base (zero width), these are spacing marks (category Lm, wcwidth = 1) that take their own halfwidth cell

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.5%. Comparing base (de35496) to head (7c3c644).

Files with missing lines	Patch %	Lines
src/PrettyPrompt/Rendering/UnicodeWidth.cs	85.7%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #294   +/-   ##
=====================================
  Coverage   84.5%   84.5%           
=====================================
  Files         47      47           
  Lines       3972    3977    +5     
  Branches     664     666    +2     
=====================================
+ Hits        3357    3363    +6     
  Misses       452     452           
+ Partials     163     162    -1     

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