feat: add symmetric mean absolute percentage error (sMAPE)

[oglego]

Fixes #436

This PR adds the Symmetric Mean Absolute Percentage Error (sMAPE) metric.

sMAPE is a regression metric that addresses the asymmetry found in standard MAPE by normalizing the absolute error by the average of the actual and predicted values. This implementation is numerically stabilized with a small epsilon to handle cases where both the ground truth and prediction are zero.

Verified sMAPE testing with

cargo test metrics_regression

I tried to keep the formatting and style consistent with MAPE but if there is anything that needs to be changed just let me know!

[codecov]

Codecov Report

❌ Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.70%. Comparing base (12c6c73) to head (18a622d).

Files with missing lines	Patch %	Lines
src/metrics_regression.rs	61.53%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
- Coverage   76.73%   76.70%   -0.03%     
==========================================
  Files         106      106              
  Lines        7410     7423      +13     
==========================================
+ Hits         5686     5694       +8     
- Misses       1724     1729       +5     

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