Reduce shift size in reduced-cost fixing

[Opt-Mucca]

This should fix #2980

The error: Reduced cost fixing is using too large of a shift when computing the lurking bounds. It should be trying a value near the start of the step range, not near the end.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.31%. Comparing base (0a9e7ca) to head (7cf21f0).
⚠️ Report is 12 commits behind head on latest.

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #2986   +/-   ##
=======================================
  Coverage   81.30%   81.31%           
=======================================
  Files         359      359           
  Lines       88792    88852   +60     
=======================================
+ Hits        72196    72250   +54     
- Misses      16596    16602    +6     

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