Faster xenium points parsing by precomputing categories

[LucaMarconato]

~20x faster performance when reading points data in Xenium (tested with around ~100 million transcripts). See scverse/spatialdata#1061

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.98%. Comparing base (785fa9a) to head (3b756da).

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/xenium.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
+ Coverage   62.94%   62.98%   +0.03%     
==========================================
  Files          27       27              
  Lines        3139     3147       +8     
==========================================
+ Hits         1976     1982       +6     
- Misses       1163     1165       +2     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/xenium.py	72.43% <77.77%> (+0.06%)	⬆️

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