refactor(internal): tidy up diffStr inner loop

[tomas-zijdemans]

Three small structural cleanups in the word-diff inner loop of diffStr

• consume bLines via an index pointer instead of shift (removes an O(n²) pattern and the in-place mutation)
• hoist the fixed aTokens = tokenize(a.value, true) above the inner while
• replace the 2-element tokenized array + .reverse() with a ternary on diff(...). Drops the as [string[], string[]] cast

Benhmarks show only modest gains.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.60%. Comparing base (0796a25) to head (a4b276a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7109   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files         633      633           
  Lines       51777    51777           
  Branches     9324     9325    +1     
=======================================
  Hits        48986    48986           
  Misses       2216     2216           
  Partials      575      575           

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