Performance improvements - span, ascii fast path, viewport optimization

[waf]

Per-change (benchmarks, 1000-line document):

• ASCII width memo table: memoized wcwidth for ASCII character ranges. 11,776 µs to 474 µs.
• Viewport-scoped highlight lookup: build the span dictionary from visible lines only, not the whole document. 826 µs to 627 µs on navigate.
• Boundary scans over cached text: replaced O(chunks) StringBuilder indexer walks with a span search (which is vectorized): 285 ns to 4.4 ns on Home/End keypresses
• GetWidth(span) ASCII fast path: skip the walking algorithm across printable-ASCII runs : 33.7 µs to 64 ns on completion-pane rendering.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.7%. Comparing base (b7bbd3d) to head (ff63588).

Files with missing lines	Patch %	Lines
src/PrettyPrompt/Highlighting/CellRenderer.cs	57.1%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #307     +/-   ##
=======================================
+ Coverage   86.6%   86.7%   +0.1%     
=======================================
  Files         48      48             
  Lines       4131    4165     +34     
  Branches     685     691      +6     
=======================================
+ Hits        3581    3615     +34     
- Misses       383     384      +1     
+ Partials     167     166      -1     

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