Improve Xenium performance, fix multinucleate cells bug

[LucaMarconato]

The last commit message is indicative of this PR. Copy-pasting it below.

Refactor xenium polygon reader to use zarr-based indices mapping
Replace slow parquet string-based cell_id grouping with zarr polygon_sets
for both nucleus and cell boundaries. This fixes a multinucleate cell bug
where multiple nuclei sharing the same cell_id were incorrectly merged
into a single polygon, and improves performance by avoiding expensive
string operations on large parquet columns.

Key changes:

• Split _get_labels_and_indices_mapping into focused functions:
  _get_labels, _get_indices_mapping_from_zarr, _get_indices_mapping_legacy
• Nucleus boundaries now use integer label_index as GeoDataFrame index
  (with cell_id as a column), correctly handling multinucleate cells
• Cell boundaries keep string cell_id as GeoDataFrame index (legacy)
• Read only needed parquet columns for faster I/O
• Use integer label_id for fast change detection when available

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.55102% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.65%. Comparing base (9011cfd) to head (c144815).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/xenium.py	77.55%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   61.80%   61.65%   -0.15%     
==========================================
  Files          27       27              
  Lines        3155     3161       +6     
==========================================
- Hits         1950     1949       -1     
- Misses       1205     1212       +7     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/xenium.py	71.04% <77.55%> (-1.44%)	⬇️

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