Fix SR-CNN missing anomalies when Period > 0

[FanBaoMS]

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Fixes #5891 - DetectEntireAnomalyBySrCnn - No anomalies detected as of version 1.5.4 or greater

Root cause

Two domain-mismatch bugs in the Period > 0 code path of SrCnnEntireModeler:

1. Train() — z-score statistics computed in the wrong domain.
_mean / _std were computed from the raw input series before deseasonality ran. The z-score "false anomaly" filter inside SpectralResidual then compared _seriesToDetect[i] (the deseasonalized residual) against those raw-domain stats, producing meaningless z-scores that suppressed real anomalies.

2. GetMarginPeriod() — CalculateAnomalyScore called with exp in the wrong domain.
_ifftRe[i] was passed as the expected-value argument. CalculateAnomalyScore computes |exp - value| / unit, which is only meaningful when both arguments share a domain.

In the no-period sibling GetMargin(), this call is correct: CalculateExpectedValueByFft(_deAnomalyData) is invoked immediately beforehand and overwrites _ifftRe with the raw-domain expected value of the de-anomalized series — so _ifftRe[i] and values[i] share a domain there.

GetMarginPeriod() was added later (#5202) and reused the same call shape without that prior overwrite. In the period > 0 path _ifftRe holds the SR saliency map's real component computed on the deseasonalized residual series, not a raw-domain expected value. The resulting score collapses to roughly |raw_value| / unit — proportional to raw magnitude rather than deviation from expectation — so real anomalies often score near zero. This looks like a copy-paste of the no-period call site without re-deriving what _ifftRe actually holds in the new path.

Fix

1. Move the sum / _mean / _std computation in Train() to after the deseasonality call and compute the statistics against _seriesToDetect (the same series SR will actually score).
2. In GetMarginPeriod() pass results[i][3] (the raw-domain expected value just produced by GetExpectedValuePeriod) instead of _ifftRe[i].

The no-period path (Train with _period == 0 and GetMargin) is correct as written and is unchanged.

Test

Adds TestSrCnnAnomalyDetectorPhoneCalls, ported from the dotnet/samples PhoneCallsAnomalyDetection tutorial. Data file is taken verbatim from dotnet/samples.

The test asserts DetectSeasonality returns 7 and that DetectEntireAnomalyBySrCnn flags exactly indices {28, 44, 56, 70}. Prior to this fix the same call detects zero anomalies on the same input.

Sensitivity = 87.0 is used to obtain the boundary width the tutorial was originally calibrated for under the v1.5.2 _factors table; the table was rewritten in #5436, so the tutorial's literal Sensitivity = 64.0 now produces a much wider boundary that masks all but the strongest spike at index 28.

[FanBaoMS]

@dotnet-policy-service agree company="Microsoft"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.60%. Comparing base (4c8b357) to head (197a859).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7610   +/-   ##
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  Coverage   69.59%   69.60%           
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  Files        1484     1484           
  Lines      273606   273641   +35     
  Branches    27949    27952    +3     
=======================================
+ Hits       190408   190457   +49     
+ Misses      75836    75822   -14     
  Partials     7362     7362           

Flag	Coverage Δ
Debug	69.60% <100.00%> (+<0.01%)	⬆️
production	63.85% <100.00%> (+<0.01%)	⬆️
test	89.64% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rosoft.ML.TimeSeries/SrCnnEntireAnomalyDetector.cs	93.51% <100.00%> (+1.14%)	⬆️
...crosoft.ML.TimeSeries.Tests/TimeSeriesDirectApi.cs	99.39% <100.00%> (+0.02%)	⬆️

... and 8 files with indirect coverage changes

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