Use smoothstep for lsqfit window blending

[elenamarkovic-dev]

Fixes #70.

This changes lsqfitatpos window blending to use the existing _smoothstep helper instead of a linear fractional weight. The smoother weight was already present as dead/commented code, so this restores the intended smooth transition between neighboring fit windows.

I also added a focused regression test that compares the public lsqfitatpos result against smoothstep-weighted left/right least-squares fits and checks that it differs from the old linear blend.

Checks:

• git diff --check
• Full Julia test suite not run locally because Julia is not installed in the execution environment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.44%. Comparing base (5ced5cc) to head (42e4879).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   57.38%   62.44%   +5.06%     
==========================================
  Files          26       26              
  Lines         901      900       -1     
==========================================
+ Hits          517      562      +45     
+ Misses        384      338      -46     

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