Add mappedVectorsFromIterable helper

[simPod]

Summary

• add mappedVectorsFromIterable() to group mapped values into Ds\\Vector instances by key
• cover the new helper in DsTest and tighten mapped collection test callbacks to satisfy static analysis
• verify the change with vendor/bin/phpunit and vendor/bin/phpstan analyse

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.93%. Comparing base (7daddbe) to head (f00b0aa).
⚠️ Report is 5 commits behind head on 0.2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##            0.2.x      #82      +/-   ##
==========================================
+ Coverage   80.43%   85.93%   +5.50%     
==========================================
  Files           6        6              
  Lines          46       64      +18     
==========================================
+ Hits           37       55      +18     
  Misses          9        9              

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