perf: Explicit transpose in new-streaming equi-join finalize

[orlp]

This ensures the Drop happens in parallel and reduces clones which were bottlenecks.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 79.10%. Comparing base (5fc9a19) to head (62efc22).
Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-stream/src/nodes/joins/equi_join.rs	0.00%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20363      +/-   ##
==========================================
- Coverage   79.10%   79.10%   -0.01%     
==========================================
  Files        1572     1572              
  Lines      219919   219958      +39     
  Branches     2465     2465              
==========================================
+ Hits       173973   173999      +26     
- Misses      45378    45391      +13     
  Partials      568      568              

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