Add mappedMapsFromIterable function

[simPod]

Summary

• Add mappedMapsFromIterable() that groups iterable into Map<K, Map<KInner, V>>, analogous to existing mappedSetsFromIterable and mappedQueuesFromIterable
• Add test covering grouping and inner key/value mapping

Test plan

• vendor/bin/phpunit tests/DsTest.php — 7 tests, 25 assertions pass
• vendor/bin/phpstan — no errors
• vendor/bin/phpcs — no violations

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.63%. Comparing base (7daddbe) to head (bd36296).
⚠️ Report is 1 commits behind head on 0.2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##            0.2.x      #77      +/-   ##
==========================================
+ Coverage   80.43%   83.63%   +3.20%     
==========================================
  Files           6        6              
  Lines          46       55       +9     
==========================================
+ Hits           37       46       +9     
  Misses          9        9              

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