perf: new obs handling default for shuffling

[ilan-gold]

1. Big chunks are better for dask
2. Reading the categoricals into memory means we don't have to go through dask for concatenation at a memory penalty (negligible from my testing)

From what I see, this should be about a ~40% reduction in shuffling time

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.45%. Comparing base (627eb08) to head (273e655).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/annbatch/io.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #117   +/-   ##
=======================================
  Coverage   92.44%   92.45%           
=======================================
  Files           6        6           
  Lines         622      636   +14     
=======================================
+ Hits          575      588   +13     
- Misses         47       48    +1     

Files with missing lines	Coverage Δ
src/annbatch/io.py	94.71% <94.44%> (-0.12%)	⬇️

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[felix0097]

LGTM!