perf: use downcasting for sparse matrices when writing to disk

[ilan-gold]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.78%. Comparing base (d658d9a) to head (7a16e64).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #129   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files          10       10           
  Lines         803      803           
=======================================
  Hits          729      729           
  Misses         74       74           

Files with missing lines	Coverage Δ
src/annbatch/io.py	93.08% <100.00%> (ø)

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