online moments

[YeungOnion]

• feat: add OnlineMoments accumulator using Welford
• fix: wrap online stats with iter_statistics for Numerical Stability of Variance #376
• fix: drop excess precision requirements

[codecov]

Codecov Report

❌ Patch coverage is 80.10204% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.67%. Comparing base (14d3f8f) to head (faaf80a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/statistics/accumulate.rs	11.53%	23 Missing ⚠️
src/statistics/online.rs	89.47%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #394      +/-   ##
==========================================
- Coverage   94.69%   94.67%   -0.02%     
==========================================
  Files          59       61       +2     
  Lines       13068    13322     +254     
==========================================
+ Hits        12375    12613     +238     
- Misses        693      709      +16     

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