fix: xNoiseSanPlot refined negative noise read from wrong array

[lpatiny]

Bug

In the refine branch of xNoiseSanPlot, the refined negative noise level was read from cloneSignPositive (the positive signal) while indexing with cloneSignNegative.length — a copy-paste error from the positive branch just above. With refine: true (the default), the reported negative level did not reflect the negative signal at all.

The fix reads from cloneSignNegative and negates it, mirroring the initial negative-noise formula (-1 * signNegative[...]). Added a regression assertion: the refined negative level is now positive and of the same order as the positive one.

All 2230 tests pass.

For your review, @jobo322 — a second, separate issue I did not touch

determineCutOff accumulates averageValue as a plain sum and then uses it as a mean in (element[1] - averageValue)², so kiSqrt isn't the intended dispersion measure and the chosen cut-off is likely skewed. Dividing averageValue by elementsOfCutOff.length looks correct, but it changes the magnitude-mode SNR on the test spectrum from ~40.2 to ~63.5 — a real behavioral change to the published-method output. I left it as-is since that needs your domain judgment on what the reference value should be. Happy to open a follow-up if you confirm the intended behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (9fed281) to head (2c12305).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #386      +/-   ##
==========================================
+ Coverage   97.13%   97.15%   +0.02%     
==========================================
  Files         207      209       +2     
  Lines        4147     4148       +1     
  Branches     1036     1030       -6     
==========================================
+ Hits         4028     4030       +2     
+ Misses        115      114       -1     
  Partials        4        4              

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[jobo322]

seems to me that cutOffSignalsIndexNeg would be always zero. because cutOffSignals defined positive. So there is not a refinning for noiseLevelNegative.

[jobo322]

Correct sign handling for negative noise in xNoiseSanPlot and uses cutoff for negative using negative data

Experiment	Refactored	Current state
DEPT
1H

[jobo322]

@lpatiny could you have a look, I propose use negative data to find the negative noise level