feat(streams/unstable): add BatchStream

[tomas-zijdemans]

This covers a pattern I use a lot in my own code, so I think it would be valuable in std.

BatchStream is a stream transform that groups incoming values into fixed-size arrays. It emits a full batch when the batch size is reached, and emits the last smaller batch when the stream ends.

Useful for grouping streamed XML records, JSON objects, CSV rows, queue messages, or other values before sending them to bulk APIs or batch processing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (0796a25) to head (a83e33e).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7110   +/-   ##
=======================================
  Coverage   94.60%   94.61%           
=======================================
  Files         633      634    +1     
  Lines       51777    51800   +23     
  Branches     9324     9328    +4     
=======================================
+ Hits        48986    49010   +24     
  Misses       2216     2216           
+ Partials      575      574    -1     

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