fix: actually set the column to the in-memory data when shuffling by default

[ilan-gold]

@felix0097 Beyond this being a bug against the original intention, this should account for the performance drop we saw (and what I saw over what I tried out on DENBI). obs['foo'] = ... != `setattr(obs, 'foo', ...) agh!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.39%. Comparing base (cbacf43) to head (a1bdcfc).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   94.22%   91.39%   -2.83%     
==========================================
  Files          10       10              
  Lines         744      744              
==========================================
- Hits          701      680      -21     
- Misses         43       64      +21     

Files with missing lines	Coverage Δ
src/annbatch/io.py	94.71% <100.00%> (ø)

... and 3 files with indirect coverage changes

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