[wangyinz]

Fixes #856.

Maintainer-approved behavior

• Smooth the SNR curve with a centered moving average over the bins that actually exist at each edge; the shortened edge window is renormalized and no zero, reflection, or extrapolated bins are introduced.
• Define the passband by strict SNR > threshold; a bin equal to the threshold is outside the passband and is returned as the boundary bin.
• Validate frequency and smoother parameters before spectrum access, clamp the requested ceiling without rounding above it, and avoid out-of-range calls for empty or unequal-length spectra.

Verification

• EstimateBandwidth contract suite: 47 passed
• Combined broadband/diagnostic/window/EstimateBandwidth contracts: 81 passed
• Existing non-database SNR numerical regressions: passed
• Black, Python 3.12/3.13 py_compile, and git diff --check: passed
• The tests include an independent manual truncated-window oracle, constant-spectrum edge preservation, exact-threshold cases, and bounded unequal-grid cases

[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.63%. Comparing base (b3bae32) to head (7cbed89).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
python/mspasspy/algorithms/snr.py	96.07%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #920      +/-   ##
==========================================
+ Coverage   70.51%   70.63%   +0.12%     
==========================================
  Files         176      177       +1     
  Lines       35094    35193      +99     
  Branches     2498     2503       +5     
==========================================
+ Hits        24745    24860     +115     
+ Misses       9449     9430      -19     
- Partials      900      903       +3     

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[wangyinz]

Draft decision required (renewed review)

A signal-processing owner must define smoothing behavior at spectral boundaries and the exact threshold-equality convention. This PR stays draft until the selected policy is documented and validated by an independent synthetic passband oracle rather than by reproducing NumPy's implementation.

[wangyinz]

Maintainer decision implemented: smoothing uses a truncated, renormalized moving window containing only real spectral bins, and the passband is strictly SNR > threshold (equality is outside). Independent edge and equality oracles pass. The former scientific blocker is resolved. This PR is temporarily stacked on #918 and is now Ready.