feat: add Matrix.gram() for the symmetric XᵀX product

[lpatiny]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.02%. Comparing base (67cda77) to head (f580a58).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
+ Coverage   64.83%   65.02%   +0.19%     
==========================================
  Files          47       47              
  Lines        5625     5656      +31     
  Branches      954      959       +5     
==========================================
+ Hits         3647     3678      +31     
  Misses       1967     1967              
  Partials       11       11              

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